Semiconductor display device and driving method the same

ABSTRACT

It is an object to provide a semiconductor display device having a touch panel, which can reduce power consumption. The semiconductor display device includes a panel which is provided with a pixel portion and a driver circuit which controls an input of the image signal to the pixel portion, and a touch panel provided in a position overlapping with the panel in the pixel portion. The pixel portion includes a display element configured to perform display in accordance with voltage of the image signal to be input, and a transistor configured to control retention of the voltage. The transistor includes an oxide semiconductor in a channel formation region. The driving frequency of the driver circuit, that is, the number of writing operations of the image signal for a certain period is changed in accordance with an operation signal from a touch panel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/094,397, filed Apr. 26, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-102891 on Apr. 28, 2010, both of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to an active-matrix semiconductor displaydevice using a transistor and a driving method thereof.

BACKGROUND ART

A touch panel is a position input device which can detect a positionindicated with a finger, a stylus, or the like and can generate a signalincluding the positional information. A display device which is obtainedin such a manner that a touch panel overlaps with an image display areais referred to as a touch screen, and the display device can display animage in an image display region and can obtain as information whichposition in the image display region a user indicates. In addition, anexample of a touch screen includes a touch screen in which aphotoelectric conversion element called a photosensor is provided in animage display region and a position indicated by a user is detected byintensity of light. The touch screen has both functions as a positioninput device and as a display device; therefore, the touch screen hashigh operability and the size of an electronic device is easily reducedas compared to the case where a position input device such as a touchpador a mouse is used.

An information display device having a touch panel and a liquid crystaldisplay panel is described in Patent Document 1.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2001-022508

DISCLOSURE OF INVENTION

As described earlier, a touch screen has an advantage of easily reducingthe size of an electronic device. A touch panel or a photosensor isadded to a thin semiconductor display device such as a flat paneldisplay, whereby the size or thickness of an electronic device can bereduced further. Therefore, semiconductor display devices to which touchpanels are added can be expected to be applied not only to stationaryelectronic devices but also to various electronic devices includingportable electronic devices.

Low power consumption is one of the important points in terms ofevaluating the performance of a semiconductor display device, and asemiconductor display device having a touch panel or a photosensor is noexception in that point. In particular, when a portable electronicdevice such as a cellular phone is used, large power consumption of asemiconductor display device having a touch panel or a photosensor leadsto a disadvantage of short continuous operation time; therefore, lowpower consumption is required.

Even in Patent Document 1, it is an object to reduce power consumption.Specifically, Patent Document 1 describes a structure in which thedriving of a liquid crystal display panel is stopped when there is notouch panel key operation so as to reduce power consumption. However, itis necessary to limit the kinds of liquid crystal materials in order toachieve the above-mentioned structure in Patent Document 1; therefore,versatility is low. In addition, display layers corresponding to theirrespective colors are stacked in the above-described liquid crystaldisplay panel in order to display a full color image; therefore, loss oflight inside the panel is large, and display is dark.

In view of the foregoing problems, it is an object of the presentinvention to provide a semiconductor display device having a touch panelor a photosensor, which can prevent quality loss of an image and canreduce power consumption. Alternatively, it is an object of the presentinvention to provide a driving method of a semiconductor display devicehaving a touch panel or a photosensor, which can prevent quality loss ofan image and can reduce power consumption.

The present inventors have thought that it is usually easier for a userto specify an input position when an image displayed in an image displayregion is a still image rather than a moving image, just before thepositional information is input to a semiconductor display device. Theyhave focused on increasing a period in which a still image is displayedin the image display region because a standby period of an input of thepositional information from the user tends to increase when thepositional information is input to the semiconductor display deviceintermittently. The inventors have thought that there is room to reducethe power consumption of the semiconductor display device in the periodin which a still image is displayed.

Thus, in a semiconductor display device according to one embodiment ofthe present invention, driving frequency when a still image is displayedjust before an input of the positional information to a touch panel or aphotosensor is lower than driving frequency when a moving image isdisplayed, whereby power consumption of the semiconductor display deviceis reduced. Alternatively, in a semiconductor display device accordingto one embodiment of the present invention, driving frequency when astill image is displayed after an input of the positional information toa touch panel or a photosensor is lower than driving frequency when amoving image is displayed, whereby power consumption of thesemiconductor display device is reduced. With the above-mentionedstructure, the power consumption can be reduced in a period in which aninput of the positional information to the touch panel or thephotosensor is in a standby period.

Further, in one embodiment of the present invention, a display elementand an insulated gate field effect transistor with extremely lowoff-state current (hereinafter referred to simply as a transistor) forcontrolling the retention of voltage applied to the display element areprovided in a pixel portion corresponding to an image display region ofa semiconductor display device in order to achieve the above-mentionedstructure. The transistor with extremely low off-state current is used,whereby the period in which voltage applied to the display element isheld can be increased. Accordingly, for example, in the case where imagesignals each having the same image information are written to a pixelportion for some consecutive frame periods, like a still image, displayof an image can be maintained even when driving frequency is low, inother words, the number of writing operations of an image signal for acertain period is reduced.

A channel formation region of the transistor includes a semiconductormaterial whose band gap is wider than that of a silicon semiconductorand whose intrinsic carrier density is lower than that of silicon. Witha channel formation region including a semiconductor material having theabove characteristics, a transistor with an extremely low off-statecurrent can be achieved. As examples of such a semiconductor material,an oxide semiconductor having a band gap which is approximately twice ormore as large as that of silicon can be given. The transistor having theabove-mentioned structure is used as a switching element used forholding voltage applied to a display element, whereby leakage of chargefrom the display element can be prevented.

Specifically, a semiconductor display device according to one embodimentof the present invention includes a panel provided with a pixel portionand a driver circuit used for controlling an input of an image signal tothe pixel portion, and a touch panel provided in a position overlappingwith the panel in the pixel portion. The pixel portion includes adisplay element which performs display in accordance with voltage of animage signal to be input and a transistor used for controlling retentionof the voltage. A channel formation region of the transistor contains asemiconductor material whose band gap is wider than that of a siliconsemiconductor and whose intrinsic carrier density is lower than that ofsilicon, such as an oxide semiconductor, for example. In addition to theabove-mentioned structure, driving frequency of the driver circuit, thatis, the number of writing operations of an image signal for a certainperiod is changed in accordance with an operation signal input from thetouch panel in the semiconductor display device according to oneembodiment of the present invention.

Alternatively, a semiconductor display device according to oneembodiment of the present invention includes a panel provided with apixel portion and a driver circuit used for controlling an input of animage signal to the pixel portion. The pixel portion includes a pixelprovided with a display element used for performing display inaccordance with voltage of an image signal to be input and a transistorused for controlling retention of the voltage. Further, the pixelportion includes a photosensor, and the photosensor includes atransistor and a light-receiving element which has a function ofgenerating an electrical signal when receiving light, such as aphotodiode. A channel formation region of the transistor contains asemiconductor material whose band gap is wider than that of a siliconsemiconductor and whose intrinsic carrier density is lower than that ofsilicon, such as an oxide semiconductor, for example. In addition to theabove-mentioned structure, driving frequency of the driver circuit, thatis, the number of writing operations of an image signal for a certainperiod is changed in accordance with an operation signal input from atouch panel in the semiconductor display device according to oneembodiment of the present invention.

Note that an oxide semiconductor is metal oxide showing semiconductorcharacteristics including both of high mobility which is almost the sameas that of microcrystalline silicon or polysilicon and uniform elementcharacteristics which are characteristics of amorphous silicon. An oxidesemiconductor highly-purified (a purified OS) by reduction inconcentration of impurities such as moisture or hydrogen, which servesas electron donors (donors), is an i-type semiconductor (an intrinsicsemiconductor) or a substantially i-type semiconductor. Therefore, atransistor including the above oxide semiconductor has a characteristicof extremely low off-state current or extremely low leakage current.Specifically, the concentration of hydrogen in the highly-purified oxidesemiconductor, which is measured by secondary ion mass spectrometry(SIMS), is 5×10¹⁹/cm³ or less, preferably 5×10¹⁸/cm³ or less, morepreferably 5×10¹⁷/cm³ or less, still more preferably less than1×10¹⁶/cm³. In addition, the carrier density of the oxide semiconductorfilm, which is measured by Hall effect measurement, is less than1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, more preferably less than1×10¹¹/cm³. Furthermore, the band gap of the oxide semiconductor is 2 eVor more, preferably 2.5 eV or more, more preferably 3 eV or more. Byusing a highly-purified oxide semiconductor film with sufficientlyreduced concentration of impurities such as moisture and hydrogen,off-state current or leakage current of the transistor can be reduced.

The analysis of the concentration of hydrogen in the oxide semiconductorfilm is described here. The concentrations of hydrogen in the oxidesemiconductor film and the conductive film are measured by SIMS. It isknown that it is difficult to obtain data in the proximity of a surfaceof a sample or in the proximity of an interface between stacked filmsformed using different materials by the SIMS in principle. Thus, in thecase where distributions of the concentrations of hydrogen in the filmsin thickness directions are analyzed by SIMS, an average value in aregion where the films are provided, the value is not greatly changed,and almost the same value can be obtained are employed as theconcentration of hydrogen. Further, in the case where the thickness ofthe film is small, a region where almost the same value can be obtainedcannot be found in some cases due to the influence of the concentrationof hydrogen in the films adjacent to each other. In this case, themaximum value or the minimum value of the concentration of hydrogen in aregion where the films are provided is employed as the concentration ofhydrogen in the film. Furthermore, in the case where a mountain-shapedpeak having the maximum value and a valley-shaped peak having theminimum value do not exist in the region where the films are provided,the value of the inflection point is employed as the concentration ofhydrogen.

Various experiments can actually prove low off-state current of thetransistor including the highly-purified oxide semiconductor film as anactive layer. For example, even when an element has a channel width of1×10⁶ μm and a channel length of 10 μm, off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at voltage (drainvoltage) between the source electrode and the drain electrode of from 1V to 10 V. In this case, it can be found that an off-state currentdensity corresponding to a value obtained by dividing the off-statecurrent by the channel width of the transistor is less than or equal to100 zA/μm. Further, an off-state current density was measured by use ofa circuit in which a capacitor and the transistor (the thickness of agate insulating film is 100 nm) are connected to each other and chargewhich is supplied to or discharged from the capacitor is controlled bythe transistor. In the measurement, the highly-purified oxidesemiconductor film was used as a channel formation region in thetransistor, and the off-state current density of the transistor wasmeasured from change in the amount of electric charge of the capacitorper unit time. As a result, it was found that in the case where thevoltage between the source electrode and the drain electrode of thetransistor was 3 V, a much lower off-state current density, which isfrom 10 zA/μm to 100 zA/μm, was able to be obtained. Therefore, in thesemiconductor display device according to one embodiment of the presentinvention, the off-state current density of the transistor including thehighly-purified oxide semiconductor film as an active layer can be lowerthan or equal to 10 zA/μm, preferably lower than or equal to 1 zA/μm, ormore preferably lower than or equal to 1 yA/μm, depending on the voltagebetween the source electrode and drain electrode. Accordingly, thetransistor including the highly-purified oxide semiconductor film as anactive layer has much lower off-state current than a transistorincluding silicon having crystallinity.

In addition, a transistor including a highly-purified oxidesemiconductor shows almost no temperature dependence of off-statecurrent. This is because the conductivity type is made to be as close toan intrinsic type as possible by removing impurities which becomeelectron donors (donors) in the oxide semiconductor to highly purify theoxide semiconductor, so that the Fermi level positions in the center ofthe forbidden band. This also results from the fact that the oxidesemiconductor has an energy gap of 3 eV or more and includes very fewthermally excited carriers. In addition, the source electrode and thedrain electrode are in a degenerated state, which is also a factor forshowing no temperature dependence. The transistor is operated mainlywith carriers which are injected from the degenerated source electrodeinto the oxide semiconductor, and the above independence of off-statecurrent in temperature can be explained by independence of the carrierdensity in temperature.

As the oxide semiconductor, a quaternary metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor, a ternary metal oxide such asan In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor, or a binarymetal oxide such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, an In—Ga—O-basedoxide semiconductor, an In—O-based oxide semiconductor, a Sn—O-basedoxide semiconductor, and a Zn—O-based oxide semiconductor can be used.Note that in this specification, for example, an In—Sn—Ga—Zn—O-basedoxide semiconductor means a metal oxide including indium (In), tin (Sn),gallium (Ga), and zinc (Zn), and there is no particular limitation onthe composition ratio. The above oxide semiconductor may includesilicon.

Moreover, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m >0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

In one embodiment of the present invention, driving frequency when astill image is display can be lower than driving frequency when a movingimage is displayed at the time of an input of the positional informationto a semiconductor display device. Therefore, a semiconductor displaydevice having a touch panel, which can prevent quality loss of an imageand can reduce power consumption, can be achieved. Further, a drivingmethod of a semiconductor display device which can prevent quality lossof an image and can reduce power consumption can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a structure of a semiconductor displaydevice.

FIG. 2 is a flow chart of operation of a semiconductor display device.

FIG. 3 is a timing diagram of operation of a semiconductor displaydevice.

FIG. 4 is a timing diagram of a driving signal and a power supplypotential.

FIGS. 5A and 5B each illustrate a structure of a shift register.

FIGS. 6A and 6B are timing diagrams of operation of a shift register.

FIG. 7 is a circuit diagram of a structure of a pixel portion.

FIG. 8 is a block diagram of a structure of a semiconductor displaydevice.

FIGS. 9A to 9D illustrate a method for manufacturing a transistor.

FIGS. 10A to 10C each illustrate a structure of a transistor.

FIG. 11 illustrates a structure of a touch panel.

FIGS. 12A and 12B each illustrate a structure of a touch panel.

FIG. 13 is a cross-sectional view of a pixel.

FIGS. 14A and 14B each illustrate a structure of a panel.

FIG. 15 is a perspective view of a structure of a semiconductor displaydevice.

FIGS. 16A to 16F each illustrate an electronic device.

FIGS. 17A and 17B each illustrate a structure of a pixel portionincluding a photosensor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and example of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the followingdescription and it is easily understood by those skilled in the art thatthe mode and details can be changed in various ways without departingfrom the spirit and scope of the present invention. Accordingly, thepresent invention should not be construed as being limited to thedescription of the embodiments and example below.

Note that the semiconductor display devices of the present inventioninclude the following in its category: liquid crystal display devices,light-emitting devices in which a light-emitting element typified by anorganic light-emitting element (OLED) is provided for each pixel,electronic paper, digital micromirror devices (DMDs), plasma displaypanels (PDPs), field emission displays (FEDs), and other semiconductordisplay devices in which a transistor is included in a pixel portion.

(Embodiment 1)

FIG. 1 is a block diagram illustrating a structure of a semiconductordisplay device according to one embodiment of the present invention.Note that in the block diagram in this specification, circuits areclassified in accordance with their functions and separated blocks areillustrated. However, it is difficult to classify actual circuitsaccording to their functions completely and it is possible for onecircuit to have a plurality of functions.

A semiconductor display device illustrated in FIG. 1 includes a panel100, a display control circuit 101, a CPU 102, and a touch panel 104.Further, the semiconductor display device according to one embodiment ofthe present invention may include a touch panel control circuit.

The panel 100 includes a pixel portion 107 provided with pixels 110 eachincluding a transistor 105 and a display element 106, and a drivercircuit 111 which controls operation of the pixel portion 107, such as asignal line driver circuit 108, a scan line driver circuit 109, and thelike. The scan line driver circuit 109 selects the pixel 110 included inthe pixel portion 107 by controlling switching of the transistor 105.The signal line driver circuit 108 controls an input of an image signalto the display element 106 of the selected pixel 110.

The display control circuit 101 controls supply of an image signal, adriving signal, and a power supply potential to the signal line drivercircuit 108 and the scan line driver circuit 109 included in the panel100. Note that although the driving signal is a signal used forcontrolling operation of the driver circuit 111 with the use of a pulse,the kind of driving signal required for the operation varies dependingon the structure of the driver circuit 111. Examples of driving signalsinclude a start signal and a clock signal which are used for controllingoperation of a shift register, and a latch signal used for controllingtiming of data retention in a memory circuit. The signal line drivercircuit 108 and the scan line driver circuit 109 can perform theabove-described operation by the supply of the driving signal and thepower supply potential.

The touch panel 104 is arranged so as to overlap with the pixel portion107 of the panel 100. When a user brings a stylus, the user's finger, orthe like into contact with the touch panel 104 or close to the vicinityof the touch panel 104, an operation signal including the positionalinformation is generated. The touch panel control circuit performsvarious kinds of signal processing on the operation signal input fromthe touch panel 104, such as AD conversion or amplitude processing, andsends the processed operation signal to the CPU 102.

The operation signal includes the positional information used foridentifying which position of the pixel portion 107 is selected by theuser. The CPU 102 uses the above-described positional informationincluded in the operation signal, and selects whether or not an image isrewritten in the pixel portion 107. Then, in accordance with the resultof the selection, the operation of the display control circuit 101 iscontrolled. Specifically, whether or not the driving signal and thepower supply potential are supplied to the driver circuit 111 isselected. In addition, for example, in the case where rewriting isperformed, an image signal corresponding to the above-describedpositional information is read from the memory circuit and transmits itto the display control circuit 101. Note that the above-described memorycircuit may be provided inside the CPU 102 or may be provided outsidethe CPU 102. Alternatively, the above-described memory circuit may beprovided outside the semiconductor display device.

Note that a correspondence relation between the position indicated inthe touch panel 104 and the position of the pixel portion 107 whichoverlaps with the position indicated in the touch panel 104 is extractedor corrected in advance by position corrective operation calledcalibration. The data of the correspondence relation is held in thememory circuit included in the CPU 102 or the memory circuit included inthe touch panel control circuit.

Note that although the structure of the semiconductor display devicewith the use of the touch panel 104 is illustrated in FIG. 1, aphotosensor instead of the touch panel 104 can be used in thesemiconductor display device according to one embodiment of the presentinvention. The photosensor together with the pixel 110 can be providedin the pixel portion 107. Unlike the case where the touch panel 104 isused, the above-described position correction operation is not alwaysrequired in the case where the photosensor is used.

In one embodiment of the present invention, after rewriting is performedby an input of the operation signal to the touch panel 104 or thephotosensor, the driving frequency of the driver circuit 111 is changedwhether an image displayed in the pixel portion 107 is a still image ora moving image. Specifically, the driving frequencies of the signal linedriver circuit 108 and the scan line driver circuit 109 when a stillimage is displayed is lower than the above-described driving frequencywhen a moving image is displayed. With the above-mentioned structure,power consumption of the semiconductor display device can be reduced.

Further, in one embodiment of the present invention, a transistor withextremely low off-state current is used in the pixel portion 107 inorder to control retention of voltage applied to the display element106. The transistor with extremely low off-state current is used,whereby the period in which voltage applied to the display element 106is held can be increased. Therefore, for example, in the case whereimage signals each having the same image information are written to thepixel portion 107 for some consecutive frame periods, like a stillimage, display of an image can be maintained even when driving frequencyis low, in other words, the number of writing operations of imagesignals to the pixel portion 107 for a certain period is reduced. Forexample, the above-described transistor in which a highly-purified oxidesemiconductor film is used as an active layer is employed, whereby aninterval between writing operations of image signals can be 10 secondsor more, preferably 30 seconds or more, further preferably 1 minute ormore. As the interval between writing operations of image signals ismade longer, power consumption can be further reduced.

Unless otherwise specified, in this specification, in the case of ann-channel (p-channel) transistor, an off-state current is a currentwhich flows between a source electrode and a drain electrode when apotential of the drain electrode is higher (lower) than that of thesource electrode or that of a gate electrode while a potential of thegate electrode is less (greater) than or equal to zero when a referencepotential is the potential of the source electrode. In addition, unlessotherwise specified, leakage current means current which flows betweenthe source electrode or the drain electrode and the gate electrodethrough an insulating film.

The operation of the semiconductor display device according to oneembodiment of the present invention can be described using the followingperiods: a period in which a moving image is displayed, and a period inwhich a still image is displayed. A specific example of operation of thepixel 110 and the driver circuit 111 when a still image is displayedwill be described with reference to FIG. 3. The time variation in anoperation state of the pixel 110 and the time variation in an operationstate of the driver circuit 111 are schematically illustrated in FIG. 3.

In the period in which a still image is displayed, a period A in whichan image signal IMG is written to the pixel 110 and a period B in whichthe display element 106 maintains the display of a gray scale by theimage signal IMG appear alternately. In FIG. 3, the case where fourperiods A of period A1 to period A4 and four periods B of period B1 toperiod B4 appear alternately is illustrated. Specifically, in FIG. 3,the periods are arranged in the following order: the period Al, theperiod B1, the period A2, the period B2, the period A3, the period B3,the period A4, and the period B4.

In each period A, the driving signal and the power supply potential aresupplied to the driver circuit 111, whereby each driver circuit such asthe signal line driver circuit 108 and the scan line driver circuit 109operates. In FIG. 3, the state in which the driver circuit 111 operatesis denoted by SST.

When the scan line driver circuit 109 is in an operation state, a scansignal SCN is input from the scan line driver circuit 109 to the pixelportion 107, whereby the pixel 110 is selected sequentially.Specifically, the transistor 105 is turned on by the scan signal SCN, sothat the pixel 110 is selected. When the signal line driver circuit 108is in an operation state, the image signal IMG is input from the signalline driver circuit 108 to the pixel 110 selected by the scan linedriver circuit 109. Specifically, the image signal IMG is input to thedisplay element 106 through the transistor 105 which is in an on state.

When the image signal IMG is input to the selected pixel 110, thedisplay element 106 displays a gray scale in accordance with the imagesignal IMG. The number of gray scales displayed by the display element106 may be a binary or may be a multi-value of three values or more. Adisplay state of the gray scale by the image signal IMG is held for acertain period.

The above-described input of the image signal IMG to the pixel 110 issimilarly performed in the other pixels 110. A display state is set inall the pixels, and an image based on data of the image signal IMG isdisplayed in the whole pixel portion 107. The state in which the data ofthe image signal IMG is written to all the pixels 110 and a displaystate is set is denoted by W in FIG. 3.

Next, in each period B, the supply of the driving signal and the powersupply potential to the driver circuit 111 is stopped, whereby eachdriver circuit such as the signal line driver circuit 108 and the scanline driver circuit 109 is in a stopped state. In FIG. 3, the state inwhich the driver circuit 111 stops operation is denoted by SSTP. Thesignal line driver circuit 108 is in a stopped state, whereby an inputof the image signal IMG to the pixel portion 107 is stopped.

In addition, the scan line driver circuit 109 is in a stopped state,whereby an input of the scan signal SCN to the pixel portion 107 isstopped. Therefore, the selection of the pixel 110 by the scan linedriver circuit 109 is stopped, so that the display element 106 includedin the pixel 110 holds a display state set in the period A just beforethe period B. The state in which the display of the gray scale by thedisplay element 106 is held is denoted by H in FIG. 3.

Specifically, in FIG. 3, a display state set in the period A1 is held inthe period B1. A display state set in the period A2 is held in theperiod B2. A display state set in the period A3 is held in the periodB3. A display state set in the period A4 is held in the period B4.

In one embodiment of the present invention, as described above, thetransistor 105 with extremely low off-state current is used; therefore,the display state in each period B can be held for 10 seconds or more,preferably 30 seconds or more, further preferably 1 minute or more.

In one embodiment of the present invention, the length of each period Bcan be changed as appropriate in accordance with timing of a pulse ofthe operation signal input to the touch panel 104 or the photosensor.For example, the case where the timing of the end of the period B2 isset by the pulse of the operation signal is illustrated in FIG. 3. InFIG. 3, the period B2 is forcibly terminated by the input of the pulseof the operation signal; then, the period A3 starts. Accordingly, in thecase of FIG. 3, the period B2 is shorter than the period B which isautomatically terminated regardless of the input of the pulse of theoperation signal, such as the period B1 and the period B3.

Note that there is a limitation on a period in which the display elementcan maintain a display state. Accordingly, in consideration of theperiod in which the display element can maintain a display state, themaximum length of each period B in a period in which the pulse of theoperation signal is not input is set in advance. That is, in the casewhere the period in which a still image is displayed is longer than themaximum length of each period B, the period B is automaticallyterminated even when there is no input of the pulse of the operationsignal. Then, the same image signal IMG is input to the pixel portion107 again in the next period A, and the image held in the period B justbefore that period A is displayed again in the whole pixel portion 107.

In one embodiment of the present invention, in a period in which a stillimage is displayed, the number of writing operations of the image signalIMG to the pixel portion 107 can be significantly reduced while thedisplay of an image is maintained. Accordingly, the driving frequency ofthe driver circuit can be drastically reduced, and the power consumptionof the semiconductor display device can be reduced.

Note that in the period in which a moving image is displayed, the imagesignal IMG is written to the selected pixel 110 in a manner similar tothat in the period in which a still image is displayed. Then, thedisplay element 106 displays a gray scale in accordance with the imagesignal IMG However, unlike the period in which a still image isdisplayed, the operation of the driver circuit is not always stoppedafter the image signal IMG is written to all the pixels 110 and thedisplay state is set.

Next, the flow of an input of the operation signal to the touch panel104 and the operation of rewriting an image in the pixel portion 107performed in accordance with the input is described. Note that in FIG.2, although the case where the touch panel 104 is used is given as anexample, similar operation can be performed even in the case where aphotosensor instead of the touch panel 104 is used.

FIG. 2 is a flow chart illustrating a flow of operation of thesemiconductor display device. In FIG. 2, before the user inputs thepositional information to the touch panel 104, the case where a stillimage is displayed in the pixel portion 107 (A01: display of a stillimage) and the case where a moving image is displayed (A02: display of amoving image) are assumed.

In one embodiment of the present invention, first, an image displayed inthe pixel portion 107 is rewritten to a still image for input used forperforming an input to the touch panel 104 (A03: shift to an inputmode). Specifically, the operation signal is input to the touch panel104 (A04: input of the operation signal to the touch panel), whereby astill image for input is displayed in the pixel portion 107 (A05:display of a still image for input). In the case where a moving image isdisplayed (A02: display of a moving image), an image is rewritten to astill image for input, whereby the user easily specifies an inputposition.

Next, based on the still image for input, the operation signal is inputto the touch panel 104 (A06: input of the operation signal to the touchpanel). The operation signal is input to the touch panel 104, whereby animage signal is written to the pixel portion 107 and an image displayedin the pixel portion 107 is rewritten. The image displayed by thisrewriting is set in accordance with the positional information includedin the operation signal. In FIG. 2, the case where a still image forinput is displayed again (A07: display of a still image for input) andthe case where an image showing information obtained by the input of theoperation signal is displayed (A08: display of a result) areillustrated. In addition, as illustrated in FIG. 2, a still image forinput may be automatically displayed again (A10: display of a stillimage for input) even when the operation signal is not input after theimage showing the information obtained by the input of the operationsignal is displayed for a certain period (A09: display of a result).

Note that an image showing the information obtained by the input of theoperation signal may be a still image or a moving image.

In one embodiment of the present invention, a driving method of stoppingthe operation of the driver circuit as illustrated in FIG. 3 is adoptedin a display period of a still image by the input of the operationsignal to the touch panel 104. In the flow chart illustrated in FIG. 2,the above-described driving method can be used for (A05: display of astill image for input), (A07: display of a still image for input), or(A10: display of a still image for input), for example.

Further, the driving method of stopping the operation of the drivercircuit as illustrated in FIG. 3 may also be adopted even in the casewhere an image showing the information obtained by the input of theoperation signal is a still image.

With the above-mentioned structure, when the user intermittently inputsthe operation signal to the touch panel 104, the operation of the drivercircuit can be stopped at the time of displaying a still image performedin an interval and power consumption can be reduced.

(Embodiment 2)

In this embodiment, in the semiconductor display device illustrated inFIG. 1, a driving signal and a power supply potential which aretransmitted from the display control circuit 101 to the driver circuit111 in a period in which a still image is displayed will be describedwith reference to FIG. 4.

A start signal SP, a clock signal CK, and a power supply potential Vpare input to the display control circuit 101. In addition, a controlsignal GDCTL and a control signal SDCTL are input from the CPU 102 tothe display control circuit 101. The control signal GDCTL is a signalfor controlling drive of the scan line driver circuit 109 and thecontrol signal SDCTL is a signal for controlling drive of the signalline driver circuit 108. The display control circuit 101 supplies asignal or potential which is input such as the start signal SP, theclock signal CK, or the power supply potential Vp to the scan linedriver circuit 109 or the signal line driver circuit 108 in accordancewith the control signal GDCTL and the control signal SDCTL.

Note that a start signal SP input to the scan line driver circuit 109 isa start signal GSP, and a start signal SP input to the signal linedriver circuit 108 is a start signal SSP. In addition, a clock signal CKinput to the scan line driver circuit 109 is a clock signal GCK, and aclock signal CK input to the signal line driver circuit 108 is a clocksignal SCK. A power supply potential Vp input to the scan line drivercircuit 109 is a power supply potential GVp, and a power supplypotential Vp input to the signal line driver circuit 108 is a powersupply potential SVp.

Note that the start signal GSP is a pulse signal corresponding to thevertical synchronization frequency, and the start signal SSP is a pulsesignal corresponding to one gate selection period.

Further, the clock signal GCK is not limited to one clock signal and aplurality of clock signals having phases different from each other maybe used as the clock signal GCK. When a plurality of clock signals isused as the clock signal GCK, operation speed of the scan line drivercircuit 109 can be improved. Further, the clock signal SCK is notlimited to one clock signal and a plurality of clock signals havingphases different from each other may be used as the clock signal SCK.When a plurality of clock signals having phases different from eachother is used as the clock signal SCK, operation speed of the signalline driver circuit 108 can be improved. Note that a common clock signalCK may be used as the clock signal GCK and the clock signal SCK.

Next, an example of a driving method of a semiconductor display deviceaccording to one embodiment of the present invention in the case wherethe above-described driving signal and power supply potential are usedis described. FIG. 4 illustrates time variation in potentials of thecontrol signal GDCTL, the power supply potential GVp, the clock signalGCK, the start signal GSP, the control signal SDCTL, the power supplypotential SVp, the clock signal SCK, and the start signal SSP. Note thatin this embodiment, as an example, the power supply potential GVp andthe power supply potential SVp are a common power supply potential, theclock signal GCK is one clock signal, the clock signal SCK is one clocksignal, and the control signal GDCTL, the control signal SDCTL, thestart signal GSP, and the start signal SSP are all binary digitalsignals.

In FIG. 4, a period can be divided into a frame period 311 in which amoving image is displayed, a frame period 312 in which a still image isdisplayed, and a frame period 313 in which a moving image is displayed.

First, in the frame period 311, the display control circuit 101 startsan output of the power supply potential GVp, the start signal GSP, andthe clock signal GCK when the pulse of the control signal GDCTL isinput. Specifically, the output of the power supply potential GVp isstarted first. After that, when the output of the power supply potentialGVp is stabilized, the output of the clock signal GCK is started; thenthe output of the start signal GSP is started. Note that the potentialof a wiring is preferably stabilized in such a manner that, just beforethe output of the clock signal GCK is started, a potential of the clocksignal GCK at high level is applied to the wiring to which the clocksignal GCK is input. By the above-described method, malfunction of thescan line driver circuit 109 in starting operation can be prevented.

Further, in the frame period 311, the display control circuit 101 startsthe output of the power supply potential SVp, the start signal SSP, andthe clock signal SCK when the pulse of the control signal SDCTL isinput. Specifically, the output of the power supply potential SVp isstarted first. After that, when the output of the power supply potentialSVp is stabilized, the output of the clock signal SCK is started; thenthe output of the start signal SSP is started. Note that the potentialof a wiring is preferably stabilized in such a manner that, just beforethe output of the clock signal SCK is started, a potential of the clocksignal SCK at high level is applied to the wiring to which the clocksignal SCK is input. By the above-described method, malfunction of thesignal line driver circuit 108 in starting operation can be prevented.

When the scan line driver circuit 109 starts operation, the scan signalSCN is input from the scan line driver circuit 109 to the scan line,whereby pixels in the pixel portion 107 are sequentially selected. Then,when the signal line driver circuit 108 starts operation, the imagesignal IMG is input from the signal line driver circuit 108 to theselected pixel through the signal line. In the pixel to which the imagesignal IMG is input, the display element sets a display state inaccordance with the image signal IMG.

Next, in the frame period 312, the display control circuit 101 stops theoutput of the power supply potential GVp, the start signal GSP, and theclock signal GCK. Specifically, the output of the start signal GSP isstopped first; then, the output of the scan signal SCN in the scan linedriver circuit 109 is stopped, so that the selection operation of allthe scan lines is terminated. Next, the output of the power supplypotential GVp is stopped. Note that “to stop an output” means, forexample, to make a wiring to which a signal or a potential is input intoa floating state, or to apply a potential at low level to a wiring towhich a signal or a potential is input. By the above-described method,malfunction of the scan line driver circuit 109 in stopping operationcan be prevented.

Note that in the frame period 312, the case where the pulse of thecontrol signal GDCTL is not input to the display control circuit 101 isillustrated in FIG. 4; however, one embodiment of the present inventionis not limited to this structure. In the frame period 312, the pulse ofthe control signal GDCTL may be input to the display control circuit101. In this case, the display control circuit 101 is acceptable as longas it is provided with a mechanism for stopping the output of the powersupply potential GVp, the start signal GSP, and the clock signal GCKeven when the pulse of the control signal GDCTL is input.

Further, in the frame period 312, the display control circuit 101 stopsthe output of the power supply potential SVp, the start signal SSP, andthe clock signal SCK. Specifically, the output of the start signal SSPis stopped first; then, the output of the image signal IMG in the signalline driver circuit 108 is stopped, so that the input operation of theimage signal IMG to all the signal lines is terminated. Then, the outputof the power supply potential SVp is stopped. By the above-describedmethod, malfunction of the signal line driver circuit 108 in stoppingoperation can be prevented.

Note that in the frame period 312, the case where the pulse of thecontrol signal SDCTL is not input to the display control circuit 101 isillustrated in FIG. 4; however, one embodiment of the present inventionis not limited to this structure. In the frame period 312, the pulse ofthe control signal SDCTL may be input to the display control circuit101. In this case, the display control circuit 101 is acceptable as longas it is provided with a mechanism for stopping the output of the powersupply potential SVp, the start signal SSP, and the clock signal SCKeven when the pulse of the control signal SDCTL is input.

Then, in the frame period 312, the display element included in the pixelholds a display state based on the data of the image signal IMG writtento the frame period 311. For example, in the case where a liquid crystalelement is used as a display element, a pixel electrode included in theliquid crystal element is in a floating state; therefore, thetransmittance set based on the data of the image signal IMG written tothe frame period 311 is held in the liquid crystal element. Therefore,in the frame period 312, the pixel portion holds an image based on thedata of the image signal IMG written to the frame period 311 as a stillimage for a certain period. Then, the length of a period for holding theimage based on the data of the image signal IMG can be controlled by,for example, pulse intervals of the control signal GDCTL and the controlsignal SDCTL which are output from the CPU 102.

Next, in a manner similar to that in the frame period 311, the displaycontrol circuit 101 starts operation of the signal line driver circuit108 and the scan line driver circuit 109 in such a manner that theoutput of the above-described driving signal and power supply potentialis started in the frame period 313.

As described in the above example, in the period in which a still imageis displayed in the semiconductor display device according to oneembodiment of this embodiment, the supply of the start signal, the clocksignal, and the power supply potential to the driver circuit can bestopped and display of an image in the pixel portion can be maintainedfor a certain period. With the above-described structure, powerconsumption of the semiconductor display device according to oneembodiment of this embodiment can be reduced.

Further, in the semiconductor display device according to one embodimentof this embodiment, an interval of writing the image signal IMG to thepixel can be increased; therefore, eye strain caused by the change ofimages can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 3)

In this embodiment, an example of a shift register which can be used ina scan line driver circuit and a signal line driver circuit of thesemiconductor display device described in the above embodiment will bedescribed.

FIGS. 5A and 5B each illustrate an example of the structure of the shiftregister in this embodiment.

The shift register illustrated in FIG. 5A is formed using P unitsequential circuits 10 (P is a natural number of 3 or more). In FIG. 5A,the P unit sequential circuits 10 are illustrated as unit sequentialcircuits FF_1 to FF_P.

A start signal ST and a reset signal Res are input to each of the unitsequential circuits FF_1 to FF_P.

Further, a clock signal CK1, a clock signal CK2, and a clock signal CK3are input to each of the unit sequential circuits FF_1 to FF_P. As theclock signal CK1, the clock signal CK2, and the clock signal CK3, forexample, any three of a first clock signal (also referred to as CLK1), asecond clock signal (also referred to as CLK2), a third clock signal(also referred to as CLK3), and a fourth clock signal (also referred toas CLK4) can be used. The first to fourth clock signals are digitalsignals whose potential level is repeatedly switched between high leveland low level. Note that the different combinations of the clock signalsare input to the adjacent unit sequential circuits 10. The shiftregister illustrated in FIG. 5A controls operation of the unitsequential circuits 10 with the first to fourth click signals. With theabove-described structure, operation speed can be improved.

Further, an example of a specific circuit configuration of the unitsequential circuit 10 illustrated in FIG. 5A is illustrated in FIG. 5B.

The unit sequential circuit illustrated in FIG. 5B includes a transistor31, a transistor 32, a transistor 33, a transistor 34, a transistor 35,a transistor 36, a transistor 37, a transistor 38, a transistor 39, atransistor 40, and a transistor 41. The case where all theabove-described transistors are n-channel transistors is given as anexample, and a specific connection relation is described below.

Note that the term “connection” in this specification refers toelectrical connection and corresponds to the state in which current,voltage, or a potential can be supplied or transmitted. Accordingly, aconnection state means not only a state of a direct connection but alsoa state of indirect connection through a circuit element such as awiring, a resistor, a diode, or a transistor so that current, voltage,or a potential can be supplied or transmitted.

Even when a circuit diagram illustrates independent components which areconnected to each other, there is the case where one conductive film hasfunctions of a plurality of components such as the case where part of awiring also functions as an electrode. The term “connection” in thisspecification includes in its category such a case where one conductivefilm has functions of a plurality of components.

The names of the “source electrode” and the “drain electrode” includedin the transistor interchange with each other depending on the polarityof the transistor or difference between the levels of potentials appliedto the respective electrodes. In general, in an n-channel transistor, anelectrode to which a lower potential is applied is called a sourceelectrode, and an electrode to which a higher potential is applied iscalled a drain electrode. Further, in a p-channel transistor, anelectrode to which a lower potential is applied is called a drainelectrode, and an electrode to which a higher potential is applied iscalled a source electrode. In this specification, one of a sourceelectrode and a drain electrode is referred to as a first terminal andthe other is referred to as a second terminal to describe the connectionrelation of the transistor.

A power supply potential Va is input to a first terminal of thetransistor 31 and a start signal ST is input to a gate electrode of thetransistor 31.

A power supply potential Vb is input to a first terminal of thetransistor 32 and a second terminal of the transistor 32 is connected toa second terminal of the transistor 31.

Note that either the power supply potential Va or the power supplypotential Vb is a high level potential Vdd, and the other is a low levelpotential Vss. In the case where all the above-described transistors arep-channel transistors, the values of the power supply potential Va andthe power supply potential Vb interchange with each other. In addition,a potential difference between the power supply potential Va and thepower supply potential Vb corresponds to power supply voltage.

A first terminal of the transistor 33 is connected to the secondterminal of the transistor 31 and the power supply potential Va is inputto a gate electrode of the transistor 33.

The power supply potential Va is input to a first terminal of thetransistor 34 and the clock signal CK3 is input to a gate electrode ofthe transistor 34.

A first terminal of the transistor 35 is connected to a second terminalof the transistor 34, a second terminal of the transistor 35 isconnected to a gate electrode of the transistor 32, and the clock signalCK2 is input to a gate electrode of the transistor 35.

The power supply potential Va is input to a first terminal of thetransistor 36 and the reset signal Res is input to a gate electrode ofthe transistor 36.

The power supply potential Vb is input to a first terminal of thetransistor 37, a second terminal of the transistor 37 is connected tothe gate electrode of the transistor 32 and a second terminal of thetransistor 36, and the start signal ST is input to a gate electrode ofthe transistor 37.

A signal to be the clock signal CK1 is input to a first terminal of thetransistor 38 and a gate electrode of the transistor 38 is connected toa second terminal of the transistor 33.

The power supply potential Vb is input to a first terminal of thetransistor 39, a second terminal of the transistor 39 is connected to asecond terminal of the transistor 38, and a gate electrode of thetransistor 39 is connected to the gate electrode of the transistor 32.

The clock signal CK1 is input to a first terminal of the transistor 40and a gate electrode of the transistor 40 is connected to the secondterminal of the transistor 33.

The power supply potential Vb is input to a first terminal of thetransistor 41, a second terminal of the transistor 41 is connected to asecond terminal of the transistor 40, and a gate electrode of thetransistor 41 is connected to the gate electrode of the transistor 32.

Note that in FIG. 5B, a point at which the second terminal of thetransistor 33, the gate electrode of the transistor 38, and the gateelectrode of the transistor 40 are connected is a node NA. A point atwhich the gate electrode of the transistor 32, the second terminal ofthe transistor 35, the second terminal of the transistor 36, the secondterminal of the transistor 37, the gate electrode of the transistor 39,and the gate electrode of the transistor 41 are connected is a node NB.A point at which the second terminal of the transistor 38 and the secondterminal of the transistor 39 are connected is a node NC. A point atwhich the second terminal of the transistor 40 and the second terminalof the transistor 41 are connected is a node ND.

The unit sequential circuit illustrated in FIG. 5B outputs a potentialof the node NC as a first output signal OUT1 and outputs a potential ofthe node ND as a second output signal OUT2. For example, the secondoutput signal OUT2 can be used as a scan signal SCN for selecting apixel in the scan line driver circuit and can be used as a signal foroutputting the image signal IMG to a selected pixel in the signal linedriver circuit.

Note that as the start signal ST input to the unit sequential circuitFF_1 of the first stage, for example, the start signal GSP, the startsignal STP, or the like in the semiconductor display device of theabove-described embodiment is used. Further, in each of the unitsequential circuits FF_2 to FF_P of the second and subsequent stages,the first output signal OUT1 in the unit sequential circuit of eachprevious stage is used as the start signal ST.

In each of the unit sequential circuits FF_1 to FF_P-2, the first outputsignal OUT1 in the unit sequential circuit which is two stages after thecurrent stage is used as the reset signal Res. In addition, in each ofthe unit sequential circuit FF_P-1 and the unit sequential circuit FF_P,a signal which is separately generated can be used as the reset signalRes, for example. Note that the unit sequential circuit FF_P-1 of the(P-1)-th stage and the unit sequential circuit FF_P of the P-th stageare each used as a dummy unit sequential circuit.

Next, the operation of the shift register illustrated in FIG. 5A will bedescribed with reference to FIGS. 6A and 6B.

FIG. 6A is a timing diagram illustrating an example of operation of theunit sequential circuit illustrated in FIG. 5B, and FIG. 6B is a timingdiagram illustrating an example of operation of the shift registerillustrated in FIG. 5A.

Note that FIG. 6A illustrates a timing diagram of the case where theunit sequential circuits 10 illustrated in FIG. 5A each have thestructure illustrated in FIG. 5B. In addition, the case where thepotential Vdd is input as the power supply potential Va and thepotential Vss is input as the power supply potential Vb when all thetransistors 31 to 41 in the unit sequential circuit 10 illustrated inFIG. 5B are n-channel transistors is given as an example and descriptionis provided below.

As illustrated in FIG. 6A, when a pulse of the start signal ST is inputto each unit sequential circuit 10 in a selection period 61, thetransistor 31 is turned on. Accordingly, a potential of the node NAbecomes larger than the potential Vdd due to the bootstrap operation,whereby the transistor 38 and the transistor 40 are turned on. Inaddition, when the transistor 37 is turned on by the input of the pulseof the start signal ST, a potential of the node NB is set to a lowlevel, whereby the transistor 39 and the transistor 41 are turned off.Accordingly, the potential of the first output signal OUT1 is set to ahigh level, and the potential of the second output signal OUT2 is set toa high level.

Further, when the transistor 36 is turned on by the input of the pulseof the reset signal Res in a non-selection period 62, the potential ofthe node NB is set to a high level, whereby the transistor 32, thetransistor 39, and the transistor 41 are turned on. In addition, whenthe transistor 32 is turned on, the potential of the node NA is set to alow level, whereby the transistor 38 and the transistor 40 are turnedoff. Accordingly, the potentials of the first output signal OUT1 and thesecond output signal OUT2 are maintained at a low level.

The above-described operation is performed sequentially in the unitsequential circuits 10 in accordance with the first clock signal CLK1 tofourth the clock signal CLK4, whereby the first output signal OUT1 andthe second output signal OUT2 whose pulses are sequentially shifted canbe output from each unit sequential circuit 10, as illustrated in FIG.6B.

In the case where the shift register described in this embodiment isused for the scan line driver circuit or the signal line driver circuitincluded in the semiconductor display device of the above embodiment,the supply of the power supply potential input to each unit sequentialcircuit, of the driving signals such as the clock signal CLK input toeach unit sequential circuit, and of the driving signals such as thestart signal SP input to the first unit sequential circuit is stopped,whereby the operation of the scan line driver circuit and the signalline driver circuit can be stopped.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 4)

In this embodiment, a liquid crystal display device which is one of thesemiconductor display devices of the present invention is given as anexample, and a specific structure of a pixel portion will be described.

A structure of a pixel portion 301 provided with a plurality of pixels300 is illustrated in FIG. 7. In FIG. 7, each of the plurality of pixels300 includes at least one of signal lines S1 to Sx and at least one ofscan lines G1 to Gy. In addition, each pixel 300 includes a transistor305 which functions as a switching element, a liquid crystal element306, and a capacitor 307. The liquid crystal element 306 includes apixel electrode, a counter electrode, and liquid crystals to whichvoltage between the pixel electrode and the counter electrode isapplied.

Each transistor 305 controls whether the potential of the signal line,namely, the potential of the image signal IMG is applied to the pixelelectrode of the liquid crystal element 306. A predetermined powersupply potential is applied to the counter electrode of the liquidcrystal element 306. In addition, the capacitor 307 includes a pair ofelectrodes; one electrode (first electrode) is connected to the pixelelectrode of the liquid crystal element 306, and a predetermined powersupply potential is applied to the other electrode (second electrode).

Note that although FIG. 7 illustrates the case where one transistor 305is used as a switching element in each pixel 300, the present inventionis not limited to this structure. A plurality of transistors may be usedas one switching element.

Next, operation of the pixel portion 301 illustrated in FIG. 7 will bedescribed.

First, when the scan lines G1 to Gy are sequentially selected, thetransistors 305 in the pixels 300 having the selected scan lines areturned on. Then, when a potential of the image signal IMG is applied tothe signal lines S1 to Sx, the potential of the image signal IMG isapplied to the pixel electrode of the liquid crystal element 306 throughthe transistors 305 which are on.

The alignment of liquid crystal molecules in the liquid crystal element306 is changed in accordance with the value of voltage applied betweenthe pixel electrode and the counter electrode, and the transmittancevaries. Accordingly, the transmittance of the liquid crystal element 306is controlled by the potential of the image signal IMG, whereby a grayscale can be displayed.

Next, when the selection of the scan lines is terminated, in the pixels300 including the scan lines, the transistors 305 are turned off. Then,voltage applied between the pixel electrode and the counter electrode isheld in the liquid crystal element 306, whereby display of a gray scaleis maintained.

Note that for a liquid crystal display device, so-called AC drivingwhich inverts the polarity of voltage to be applied to the liquidcrystal element 306 in a predetermined timing is employed in order toprevent deterioration called burn-in of liquid crystals. Specifically,AC driving can be performed in such a way that the polarity of thepotential of the image signal IMG input to each pixel 300 is reversedbased on the potential of the counter electrode as a reference. When ACdriving is performed, a change of a potential to be applied to thesignal line becomes large; therefore, a potential difference between thesource electrode and the drain electrode of the transistor 305functioning as a switching element becomes large. Accordingly, thetransistor 305 easily causes deterioration of characteristics such as ashift of threshold voltage. In addition, in order to maintain voltageheld in the liquid crystal element 306, low off-state current isrequired even when the potential difference between the source electrodeand the drain electrode is large.

In one embodiment of the present invention, a semiconductor whose bandgap is larger than that of silicon or germanium and whose intrinsiccarrier density is lower that of silicon or germanium, such as an oxidesemiconductor, is used for the transistor 305; therefore, the pressureresistance of the transistor 305 can be increased. Accordingly, thepressure resistance of the transistor 305 is increased, wherebyreliability of the liquid crystal display device can be improved.

An oxide semiconductor highly-purified by reduction of impurities suchas moisture or hydrogen which serves as an electron donor (donor) (apurified OS) is an i-type semiconductor (an intrinsic semiconductor) ora substantially i-type semiconductor. Therefore, when theabove-described oxide semiconductor is used for the transistor 305, theoff-state current of the transistor 305 can be dramatically reduced.

The off-state current of the transistor 305 is reduced, so that a changeof transmittance due to off-state current can be suppressed even whenthe number of writing operations of the image signal IMG is reduced in aperiod in which a still image is displayed, therefore, display of animage can be maintained.

Note that in one embodiment of the present invention, the potential ofthe counter electrode of the liquid crystal element 306 or the potentialof the second electrode of the capacitor 307 may be held using anothertransistor with extremely low off-state current in a period in which astill image is displayed. With the above-described structure, the powerconsumption of the semiconductor display device can be further reduced.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 5)

In this embodiment, a structure of a driver circuit included in asemiconductor display device will be described.

FIG. 8 is a block diagram illustrating an example of a more detailedstructure of the panel 100 included in a semiconductor display device.In the panel 100 illustrated in FIG. 8, the signal line driver circuit108 includes a shift register 130, a first memory circuit 131, a secondmemory circuit 132, a level shifter 133, a DAC 134, and an analog buffer135. In addition, the scan line driver circuit 109 includes a shiftregister 136 and a digital buffer 137.

Next, the operation of the panel 100 illustrated in FIG. 8 will bedescribed. At the time of the operation of the signal line drivercircuit 108, the power supply potential SVp is input to each of theabove-described circuits included in the signal line driver circuit 108.In addition, at the time of the operation of the scan line drivercircuit 109, the power supply potential GVp is input to each of theabove-described circuits included in the scan line driver circuit 109.Note that each of the power supply potential SVp and the power supplypotential GVp does not always mean one kind of a power supply potentialbut also means plural kinds of power supply potentials which havedifferent levels.

When the start signal SSP and the clock signal SCK are input to theshift register 130, the shift register 130 generates a timing signalwhose pulse sequentially is shifted.

The image signal IMG is input to the first memory circuit 131. Then,when the timing signal is input to the first memory circuit 131, theimage signal IMG is sampled in accordance with a pulse of the timingsignal to be sequentially written to a plurality of memory elementsincluded in the first memory circuit 131. That is, the image signal IMGwhich is input to the signal line driver circuit 108 in series iswritten to the first memory circuit 131 in parallel. The image signalIMG written to the first memory circuit 131 is held.

Note that the image signals IMG may be sequentially written to aplurality of memory elements included in the first memory circuit 131;or so-called division driving may be performed, in which the memoryelements included in the first memory circuit 131 are divided intoseveral groups and the image signals IMG are input to each group inparallel. Note that the number of groups in this case is referred to asthe number of divisions. For example, in the case where a memory circuitis divided into groups such that each group has four memory elements,division driving is performed with four divisions.

A latch signal LP is input to the second memory circuit 132. Afterwriting of the image signal IMG to the first memory circuit 131 iscompleted, the image signal IMG held in the first memory circuit 131 iswritten to and held in the second memory circuit 132 all at once inaccordance with a pulse of the latch signal LP input to the secondmemory circuit 132 in a retrace period. Again, in accordance with thetiming signal from the shift register 130, the next image signals IMGare sequentially written to the first memory circuit 131 in whichtransmission of the image signal IMG to the second memory circuit 132 iscompleted. In the one line period of the second round, the image signalIMG which is written to and held in the second memory circuit 132 istransmitted to the DAC 134 after the amplitude of the voltage isadjusted in the level shifter 133. In the DAC 134, the image signal IMGwhich is input is converted from a digital signal to an analog signal.Then, the image signal IMG which is converted to an analog signal istransmitted to the analog buffer 135. The image signal IMG transmittedfrom the DAC 134 is transmitted from the analog buffer 135 to the pixelportion 107 through the signal line.

In contrast, in the scan line driver circuit 109, when the start signalGSP and the clock signal GCK are input to the shift register 136, thescan signal SCN whose pulse sequentially is shifted is generated. Thescan signal SCN output from the shift register 130 is transmitted fromthe digital buffer 137 to the pixel portion 107 through the scan line.

The pixel 110 included in the pixel portion 107 is selected by the scansignal SCN input from the scan line driver circuit 109. The image signalIMG transmitted from the signal line driver circuit 108 to the pixelportion 107 through the signal line is input to the above-describedselected pixel.

In the panel 100 illustrated in FIG. 8, the start signal SSP, the clocksignal SCK, the latch signal LP, and the like correspond to the drivingsignals of the signal line driver circuit 108. In addition, the startsignal GSP, the clock signal GCK, and the like correspond to the drivingsignals of the scan line driver circuit 109. In a period in which astill image is displayed, the supply of the driving signals and thepower supply potential is stopped, whereby the number of writingoperations of the image signal IMG to the pixel portion 107 can bereduced, and power consumption of the semiconductor display device canbe reduced.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 6)

Next, an example of a method for manufacturing a transistor will bedescribed.

First, as illustrated in FIG. 9A, a gate electrode 801 and an electrode802 for a capacitor are formed over a substrate 800 having an insulatingsurface.

The gate electrode 801 and the electrode 802 can be formed with a singlelayer or a stacked layer using one or more of conductive films using ametal material such as molybdenum, titanium, chromium, tantalum,tungsten, neodymium, or scandium, or an alloy material which includesany of these metal materials as a main component, or nitride of thesemetals. Note that aluminum or copper can also be used as such a metalmaterial if it can withstand the temperature of heat treatment to beperformed in a later process. Aluminum or copper is preferably combinedwith a refractory metal material so as to prevent a heat resistanceproblem and a corrosive problem. As the refractory metal material,molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium,or the like can be used.

For example, as a two-layer structure of the gate electrode 801 and theelectrode 802, the following structures are preferable: a two-layerstructure in which a molybdenum film is formed over an aluminum film, atwo-layer structure in which a molybdenum film is formed over a copperfilm, a two-layer structure in which a titanium nitride film or atantalum nitride film is formed over a copper film, and a two-layerstructure in which a titanium nitride film and a molybdenum film arestacked. As a three-layer structure of the gate electrode 801 and theelectrode 802, the following structure is preferable: a layeredstructure in which an aluminum film, an alloy film of aluminum andsilicon, an alloy film of aluminum and titanium, or an alloy film ofaluminum and neodymium is used as a middle layer and sandwiched betweentwo films selected from a tungsten film, a tungsten nitride film, atitanium nitride film, and a titanium film is used as a top layer and abottom layer.

Further, a light-transmitting oxide conductive film of indium oxide, analloy of indium oxide and tin oxide, an alloy of indium oxide and zincoxide, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zincgallium oxide, or the like can be used as the gate electrode 801 and theelectrode 802.

The thickness of each of the gate electrode 801 and the electrode 802 is10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, afterthe conductive film for the gate electrode is formed to have a thicknessof 150 nm by a sputtering method using a tungsten target, the conductivefilm is processed (patterned) into a desired shape by etching, wherebythe gate electrode 801 and the electrode 802 are formed. Note that whenend portions of the formed gate electrode are tapered, coverage with agate insulating film formed thereover is improved, which is preferable.Note that a resist mask may be formed by an inkjet method. Formation ofthe resist mask by an inkjet method needs no photomask; thus,manufacturing cost can be reduced.

Next, as illustrated in FIG. 9B, a gate insulating film 803 is formedover the gate electrode 801 and the electrode 802. The gate insulatingfilm 803 can be formed to have a single-layer structure or a layeredstructure of one or more films selected from a silicon oxide film, asilicon nitride film, a silicon oxynitride film, a silicon nitride oxidefilm, an aluminum oxide film, an aluminum nitride film, an aluminumoxynitride film, an aluminum nitride oxide film, a hafnium oxide film,and a tantalum oxide film by a plasma-enhanced CVD method, a sputteringmethod, or the like. It is preferable that the gate insulating film 803includes impurities such as moisture, hydrogen, or oxygen as little aspossible. In the case of forming a silicon oxide film by a sputteringmethod, a silicon target or a quartz target is used as a target, andoxygen or a mixed gas of oxygen and argon is used as a sputtering gas.

The oxide semiconductor which becomes i-type or becomes substantiallyi-type (a highly-purified oxide semiconductor) due to removal ofimpurities is extremely sensitive to an interface state or an interfaceelectric charge; therefore, an interface between the highly-purifiedoxide semiconductor and the gate insulating film 803 is important.Therefore, the gate insulating film (GI) that is in contact with thehighly-purified oxide semiconductor needs to have higher quality.

For example, a high-density plasma-enhanced CVD using a microwave(frequency: 2.45 GHz) is preferably used, in which case an insulatingfilm which is dense, has high breakdown voltage, and is of high qualitycan be formed. The highly-purified oxide semiconductor and thehigh-quality gate insulating film are in close contact with each other,whereby the interface state can be reduced and interface characteristicscan be improved.

Needless to say, a different deposition method such as a sputteringmethod or a plasma-enhanced CVD method can be used as long as ahigh-quality insulating film can be formed as a gate insulating film.Moreover, it is possible to form an insulating film whose quality andcharacteristics of an interface with the oxide semiconductor areimproved through heat treatment performed after the formation of theinsulating film. In any case, an insulating film that has favorable filmquality as the gate insulating film and can reduce interface statedensity between the gate insulating film and the oxide semiconductor toform a favorable interface is formed.

The gate insulating film 803 may have a structure in which an insulatingfilm formed using a material having a high barrier property and aninsulating film having low proportion of nitrogen such as a siliconoxide film or a silicon oxynitride film are stacked. In this case, theinsulating film such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating film having a high barrierproperty and the oxide semiconductor film. As the insulating film havinga high barrier property, a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, an aluminum nitride oxide film, and thelike can be given, for example. The insulating film having a highbarrier property is used, so that impurities in an atmosphere, such asmoisture or hydrogen, or impurities included in the substrate, such asan alkali metal or a heavy metal, can be prevented from entering theoxide semiconductor film, the gate insulating film 803, or the interfacebetween the oxide semiconductor film and another insulating film and thevicinity thereof. In addition, the insulating film having lowerproportion of nitrogen such as a silicon oxide film or a siliconoxynitride film is formed so as to be in contact with the oxidesemiconductor film, so that the insulating film having a high barrierproperty can be prevented from being in contact with the oxidesemiconductor film directly.

For example, a stacked-layer film with a thickness of 100 nm may beformed as the gate insulating film 803 as follows: a silicon nitridefilm (SiN_(y) (y >0)) with a thickness of greater than or equal to 50 nmand less than or equal to 200 nm is formed by a sputtering method as afirst gate insulating film, and a silicon oxide film (SiO_(x) (x >0))with a thickness of greater than or equal to 5 nm and less than or equalto 300 nm is stacked over the first gate insulating film as a secondgate insulating film. The thickness of the gate insulating film 803 maybe set as appropriate depending on characteristics needed for atransistor and may be approximately 350 nm to 400 nm.

In this embodiment, the gate insulating film 803 is formed to have astructure in which a 100-nm-thick silicon oxide film formed by asputtering method is formed over a 50-nm-thick silicon nitride filmformed by a sputtering method.

Note that in order that the gate insulating film 803 contains as littlehydrogen, hydroxyl, and moisture as possible, it is preferable thatimpurities adsorbed on the substrate 800, such as moisture or hydrogen,be eliminated and removed by preheating the substrate 800, over whichthe gate electrode 801 and the electrode 802 are formed, in a preheatingchamber of a sputtering apparatus, as a pretreatment for film formation.The temperature for the preheating is higher than or equal to 100° C.and lower than or equal to 400° C., preferably, higher than or equal to150° C. and lower than or equal to 300° C. As an exhaustion unitprovided in the preheating chamber, a cryopump is preferable. Note thatthis preheating treatment can be omitted.

Next, over the gate insulating film 803, an oxide semiconductor filmhaving a thickness of greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 3 nm and less thanor equal to 50 nm, more preferably greater than or equal to 3 nm andless than or equal to 20 nm is formed. The oxide semiconductor film isformed by a sputtering method using an oxide semiconductor target.Moreover, the oxide semiconductor film can be formed by a sputteringmethod under a rare gas (e.g., argon) atmosphere, an oxygen atmosphere,or a mixed atmosphere of a rare gas (e.g., argon) and oxygen.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust attached to a surface of the gate insulating film 803 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, an RFpower source is used for application of voltage to a substrate side inan argon atmosphere to modify a surface. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used. Alternatively, an argon atmosphere to which oxygen, nitrousoxide, or the like is added may be used. Alternatively, an argonatmosphere to which chlorine, carbon tetrafluoride, or the like is addedmay be used.

As described above, as the oxide semiconductor film, the following oxidesemiconductors can also be used: a quaternary metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor; a ternary metal oxide such asan In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor; a binarymetal oxide such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, an In—Ga—O-basedoxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; and a Zn—O-based oxide semiconductor. Theabove-described oxide semiconductor may include silicon.

Moreover, oxide semiconductors can be represented by the chemicalformula, InMO₃(ZnO)_(m) (m >0). Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basednon-single-crystal film with a thickness of 30 nm, which is obtained bya sputtering method using a metal oxide target including indium (In),gallium (Ga), and zinc (Zn), is used. As the above-described target, atarget having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]can be used. Alternatively, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] or a target having a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] can be used, for example.The target may contain SiO₂ at greater than or equal to 2 wt % and lessthan or equal to 10 wt %. The filling factor of the metal oxide targetcontaining In, Ga, and Zn is higher than or equal to 90% and lower thanor equal to 100%, and preferably higher than or equal to 95% and lowerthan or equal to 99.9%. With the use of a metal oxide target with highfilling factor, the deposited oxide semiconductor film has high density.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, a target used has a composition ratio of In:Zn=50:1 to1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 2:1in a molar ratio), further preferably In:Zn=1.5:1 to 15:1 (In₂O₃:ZnO=3:4to 15:2 in a molar ratio). For example, when a target used for formingthe In—Zn—O-based oxide semiconductor has a composition ratio ofIn:Zn:O=X:Y:Z in an atomic ratio, Z >(1.5X+Y).

In this embodiment, the oxide semiconductor film is formed over thesubstrate 800 in such a manner that the substrate is held in thetreatment chamber kept at reduced pressure, a sputtering gas from whichhydrogen and moisture have been removed is introduced into the treatmentchamber while moisture remaining therein is removed, and theabove-described target is used. The substrate temperature may be higherthan or equal to 100° C. and lower than or equal to 600° C., preferablyhigher than or equal to 200° C. and lower than or equal to 400° C. infilm formation. By forming the oxide semiconductor film in a state wherethe substrate is heated, the concentration of impurities included in theformed oxide semiconductor film can be reduced. In addition, damage bysputtering can be reduced. In order to remove moisture remaining in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom, a compound containing a hydrogenatom, such as water (H₂O), (more preferably, also a compound containinga carbon atom), and the like are removed, whereby the concentration ofimpurities contained in the oxide semiconductor film formed in thedeposition chamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulsed direct-current (DC) power source is preferable becausedust (also referred to as particles) generated in film deposition can bereduced and the film thickness can be uniform.

In order that the oxide semiconductor film contains as little hydrogen,hydroxyl, and moisture as possible, it is preferable that impuritiesadsorbed on the substrate 800, such as moisture or hydrogen, beeliminated and removed by preheating the substrate 800, on which theprocess up to and including the step of forming the gate insulating film803 is already performed, in a preheating chamber of a sputteringapparatus, as a pretreatment for film formation. The temperature for thepreheating is higher than or equal to 100° C. and lower than or equal to400° C., preferably, higher than or equal to 150° C. and lower than orequal to 300° C. As an exhaustion unit provided in the preheatingchamber, a cryopump is preferable. Note that this preheating treatmentcan be omitted. In addition, before an insulating film 808 is formed,the preheating may similarly be performed on the substrate 800 on whichthe process up to and including the step of forming a source electrode805, a drain electrode 806, and an electrode 807 for a capacitor isalready performed.

Next, as illustrated in FIG. 9B, the oxide semiconductor film isprocessed (patterned) into a desired shape by etching or the like,whereby an island-shaped oxide semiconductor film 804 is formed over thegate insulating film 803 in a position where the island-shaped oxidesemiconductor film 804 overlaps with the gate electrode 801.

A resist mask for forming the island-shaped oxide semiconductor film 804may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

Note that etching for forming the island-shaped oxide semiconductor film804 may be wet etching, dry etching, or both dry etching and wetetching. As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused. Alternatively, a gas containing fluorine (fluorine-based gas suchas carbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, ITO-07N (produced by KANTO CHEMICALCO., INC.) may be used. The etchant after the wet etching is removedtogether with the etched materials by cleaning. The waste liquidincluding the etchant and the material etched off may be purified andthe material may be reused. When a material such as indium included inthe oxide semiconductor film is collected from the waste liquid afterthe etching and reused, the resources can be efficiently used and thecost can be reduced.

Note that it is preferable that reverse sputtering be performed beforethe formation of a conductive film in a subsequent step so that a resistresidue or the like that attaches to surfaces of the island-shaped oxidesemiconductor film 804 and the gate insulating film 803 is removed.

Then, heat treatment is performed on the oxide semiconductor film 804 ina nitrogen atmosphere, an oxygen atmosphere, an atmosphere of ultra-dryair, or a rare gas (argon, helium, or the like) atmosphere. It ispreferable that the content of water in the gas be 20 ppm or less,preferably 1 ppm or less, and more preferably 10 ppb or less. Heattreatment performed on the oxide semiconductor film 804 can eliminatemoisture or hydrogen in the oxide semiconductor film 804. Specifically,heat treatment may be performed at higher than or equal to 300° C. andlower than or equal to 850° C. (or the strain point of the glasssubstrate), preferably higher than or equal to 550° C. and lower than orequal to 750° C. For example, heat treatment may be performed at 600° C.for a period longer than or equal to 3 minutes and shorter than or equalto 6 minutes. With an RTA method for the heat treatment, dehydration ordehydrogenation can be performed in a short time; therefore, treatmentcan be performed even at a temperature higher than the strain point of aglass substrate. Alternatively, heat treatment may be performed forapproximately one hour in a state where substrate temperature isapproximately 450° C.

In this embodiment, the island-shaped oxide semiconductor film 804 issubjected to the heat treatment in a nitrogen atmosphere with the use ofan electric furnace which is one example of a heat treatment apparatus.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include an apparatus for heating an object to beprocessed by heat conduction or heat radiation from a heating elementsuch as a resistance heating element. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react withan object to be processed by heat treatment, such as nitrogen or a raregas such as argon is used.

For example, as the heat treatment, GRTA in which the substrate is movedinto an inert gas heated at a high temperature of 650° C. to 700° C.,heated for several minutes, and moved out of the inert gas heated to thehigh temperature may be performed. With GRTA, high-temperature heattreatment for a short period of time can be achieved.

Note that it is preferable that in the heat treatment, moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. It is preferable that the purity of nitrogen orthe rare gas such as helium, neon, or argon which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the concentration ofimpurities is 1 ppm or lower, preferably 0.1 ppm or lower).

When impurities such as moisture or hydrogen are added to an oxidesemiconductor, in a gate bias-temperature stress test (BT test, the testcondition is, for example, at 85° C. with 2×10⁶ V/cm for 12 hours), abond between the impurities and a main component of the oxidesemiconductor is broken by a high electrical field (B: bias) and hightemperature (T: temperature), and dangling bonds generated cause driftof the threshold voltage (Vth). However, as described above,characteristics in an interface between the gate insulating film and theoxide semiconductor film are improved and impurities in the oxidesemiconductor film, particularly moisture, hydrogen, and the like areremoved as much as possible so that a transistor which withstands a BTtest can be obtained.

Through the above-described steps, the concentration of hydrogen in theoxide semiconductor film 804 can be reduced and the oxide semiconductorfilm can be highly purified. Thus, the oxide semiconductor film can bestabilized. In addition, heat treatment at a temperature of lower thanor equal to the glass transition temperature makes it possible to forman oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, a transistor can be manufacturedusing a large-sized substrate, so that productivity can be increased. Inaddition, by using the oxide semiconductor film in which theconcentration of hydrogen is reduced and purity is improved, it ispossible to manufacture a transistor with high breakdown voltage, areduced short-channel effect, and a high on-off ratio.

Note that when the oxide semiconductor film is heated, a plate-likecrystal is formed in the upper surface though it depends on a materialof the oxide semiconductor film and heating conditions in some cases.The plate-like crystals are preferably single crystal bodies which arec-axis-aligned in a direction substantially perpendicular to a surfaceof the oxide semiconductor film. Even if the plate-like crystals are notsingle crystal bodies, each crystal is preferably a polycrystalline bodywhich is c-axis-aligned in a direction substantially perpendicular tothe surface of the oxide semiconductor film. Further, it is preferablethat the polycrystalline bodies be c-axis-aligned and that the a-bplanes of crystals correspond, or the a-axis or the b-axis of thecrystals be aligned with each other. Note that when a base surface ofthe oxide semiconductor film is uneven, a plate-like crystal is apolycrystal. Therefore, the base surface is preferably as even aspossible.

Then, a conductive film used for the source electrode or the drainelectrode (including a wiring formed in the same layer as the sourceelectrode or the drain electrode) is formed over the oxide semiconductorfilm 804 by a sputtering method or a vacuum evaporation method; then,the conductive film is patterned by etching or the like, whereby asillustrated in FIG. 9C, the source electrode 805 and the drain electrode806 are formed over the oxide semiconductor film 804, and the electrode807 which overlaps with the electrode 802 with the gate insulating film803 interposed therebetween.

As the material of the conductive film to be the source electrode 805,the drain electrode 806, and the electrode 807 (including a wiringformed in the same layer as the source electrode 805, the drainelectrode 806, and the electrode 807), there are an element selectedfrom Al, Cr, Cu, Ta, Ti, Mo, and W; an alloy including any of the aboveelements as a component; an alloy including any of these elements incombination; and the like. In addition, a structure in which a film of arefractory metal such as Cr, Ta, Ti, Mo, or W is formed on a lower sideor an upper side of a metal film of Al, Cu, or the like may be used.Furthermore, an Al material to which an element which preventsgeneration of hillocks or whisker in an Al film, such as Si, Ti, Ta, W,Mo, Cr, Nd, Sc, or Y is added may be used, leading to improvement inheat resistance.

Further, the conductive film may have a single-layer structure or alayered structure of two or more layers. For example, a single-layerstructure of an aluminum film including silicon, a two-layer structurein which a titanium film is formed over an aluminum film, a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in that order, and the like can be given.

Alternatively, the conductive film to be the source electrode 805, thedrain electrode 806, and the electrode 807 (including a wiring formed inthe same layer as the source electrode 805, the drain electrode 806, andthe electrode 807) may be formed using a conductive metal oxide. As aconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂,abbreviated to ITO), an alloy of indium oxide and zinc oxide(In₂O₃—ZnO), or the metal oxide material to which silicon or siliconoxide is added can be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor film 804 is not removed inetching of the conductive film as much as possible. Depending on etchingconditions, an exposed portion of the island-shaped oxide semiconductorfilm 804 may be partly etched, so that a groove (a recessed portion) isformed in some cases.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask through which light is transmittedso as to have a plurality of intensities. A resist mask formed with theuse of a multi-tone mask has a plurality of thicknesses and further canbe changed in shape by etching; therefore, the resist mask can be usedin a plurality of etching steps for processing into different patterns.Therefore, a resist mask corresponding to at least two kinds or more ofdifferent patterns can be formed by one multi-tone mask. Thus, thenumber of light-exposure masks can be reduced and the number ofcorresponding photolithography steps can be also reduced, wherebysimplification of a process can be achieved.

Next, plasma treatment is performed thereon, using a gas such as N₂O,N₂, or Ar. By the plasma treatment, water or the like which attaches oris adsorbed to an exposed surface of the oxide semiconductor film isremoved. Plasma treatment may be performed using a mixed gas of oxygenand argon as well.

After the plasma treatment, as illustrated in FIG. 9D, the insulatingfilm 808 is formed so as to cover the source electrode 805, the drainelectrode 806, the electrode 807, and the oxide semiconductor film 804.The insulating film 808 preferably contains impurities such as moistureor hydrogen as little as possible, and may be formed using asingle-layer insulating film or a plurality of insulating films stacked.When hydrogen is contained in the insulating film 808, entry of thehydrogen to the oxide semiconductor film or extraction of oxygen in theoxide semiconductor film by the hydrogen occurs, whereby a back channelportion of the oxide semiconductor film has lower resistance (n-typeconductivity); thus, a parasitic channel might be formed. Therefore, itis preferable that a formation method in which hydrogen is not used isemployed in order to form the insulating film 808 containing as littlehydrogen as possible. A material having a high barrier property ispreferably used for the insulating film 808. For example, as theinsulating film having a high barrier property, a silicon nitride film,a silicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like can be used. When a plurality ofinsulating films stacked is used, an insulating film having a lowerproportion of nitrogen such as a silicon oxide film or a siliconoxynitride film is formed on the side closer to the oxide semiconductorfilm 804 than the insulating film having a high barrier property. Then,the insulating film having a high barrier property is formed so as tooverlap with the source electrode 805, the drain electrode 806, and theoxide semiconductor film 804 with the insulating film having lowerproportion of nitrogen between the insulating film having a barrierproperty and the source electrode 805, the drain electrode 806, and theoxide semiconductor film 804. By using the insulating film having a highbarrier property, the impurities such as moisture or hydrogen can beprevented from entering the oxide semiconductor film 804, the gateinsulating film 803, or the interface between the oxide semiconductorfilm 804 and another insulating film and the vicinity thereof. Inaddition, the insulating film having lower proportion of nitrogen suchas a silicon oxide film or a silicon oxynitride film is formed so as tobe in contact with the oxide semiconductor film 804, so that theinsulating film formed using a material having a high barrier propertycan be prevented from being in contact with the oxide semiconductor film804 directly.

In this embodiment, the insulating film 808 having a structure in whicha silicon nitride film having a thickness of 100 nm formed by asputtering method is formed over a silicon oxide film having a thicknessof 200 nm formed by a sputtering method is formed. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C.

Note that after the insulating film 808 is formed, heat treatment may beperformed. The heat treatment is performed in an atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas (argon, helium, or the like)preferably at a temperature higher than or equal to 200° C. and lowerthan or equal to 400° C., for example, higher than or equal to 250° C.and lower than or equal to 350° C. It is preferable that the content ofwater in the gas be 20 ppm or less, preferably 1 ppm or less, and morepreferably 10 ppb or less. In this embodiment, for example, heattreatment at 250° C. in a nitrogen atmosphere for one hour is performed.Alternatively, RTA treatment for a short time at a high temperature maybe performed before the formation of the source electrode 805, the drainelectrode 806, and the electrode 807 in a manner similar to the heattreatment performed on the oxide semiconductor film. Even when oxygendeficiency occurs in the oxide semiconductor film 804 due to the heattreatment performed on the oxide semiconductor film, the insulating film808 containing oxygen is provided in contact with an exposed region ofthe oxide semiconductor film 804 provided between the source electrode805 and the drain electrode 806, and then heat treatment is performed,whereby oxygen is supplied to the oxide semiconductor film 804.Therefore, when oxygen is supplied to the region of the oxidesemiconductor film 804 which is in contact with the insulating film 808,oxygen deficiency serving as a donor can be reduced and thestoichiometric composition ratio can be satisfied. As a result, theoxide semiconductor film 804 can be made to be an i-type semiconductorfilm or a substantially i-type semiconductor film. Accordingly,electrical characteristics of the transistor can be improved andvariation in the electrical characteristics thereof can be reduced. Thetiming of this heat treatment is not particularly limited as long as itis after the formation of the insulating film 808, and this heattreatment can be performed without increasing the number ofmanufacturing steps by doubling as another step such as heat treatmentfor formation of a resin film or heat treatment for reduction of theresistance of a transparent conductive film, so that the oxidesemiconductor film 804 can be made to be an i-type semiconductor film ora substantially i-type semiconductor film.

Next, after a conductive film is formed over the insulating film 808,the conductive film is patterned, so that a back gate electrode may beformed so as to overlap with the oxide semiconductor film 804. When theback gate electrode is formed, an insulating film is formed so as tocover the back gate electrode. The back gate electrode can be formedusing a material and a structure similar to those of the gate electrode801, the electrode 802, the source electrode 805 and the drain electrode806, or the electrode 807.

The thickness of the back gate electrode is set to be 10 nm to 400 nm,preferably 100 nm to 200 nm. In this embodiment, the back gate electrodemay be formed in a such a manner that a conductive film in which atitanium film, an aluminum film, and a titanium film are stacked isformed, a resist mask is formed by a photolithography method or thelike, and unnecessary portions are removed by etching so that theconductive film is processed (patterned) into a desired shape.

The insulating film is preferably formed using a material having a highbarrier property which can prevent moisture, hydrogen, and the like inan atmosphere from influencing characteristics of the transistor. Forexample, the insulating film can be formed to have a single-layerstructure or a layered structure of a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, an aluminum nitride oxidefilm, or the like, as an insulating film having a high barrier property,by a plasma-enhanced CVD method, a sputtering method, or the like. Inorder to obtain an effect of a barrier property, the insulating film ispreferably formed to a thickness of 15 nm to 400 nm, for example.

In this embodiment, an insulating film is formed to a thickness of 300nm by a plasma-enhanced CVD method. The insulating film is formed underthe following conditions: the flow rate of a silane gas is 4 sccm; theflow rate of dinitrogen monoxide (N₂O) is 800 sccm; and the substratetemperature is 400° C.

With the above steps, a transistor 809 and a capacitor 810 are formed.Note that the capacitor 810 is formed in a region where the electrode802 overlaps with the electrode 807 with the gate insulating film 803interposed therebetween.

The transistor 809 includes the gate electrode 801, the gate insulatingfilm 803 over the gate electrode 801, the oxide semiconductor film 804which is over the gate insulating film 803 and overlaps with the gateelectrode 801, and the source electrode 805 and the drain electrode 806which are a pair and formed over the oxide semiconductor film 804. Thetransistor 809 may further include the insulating film 808 provided overthe oxide semiconductor film 804 as a component. The transistor 809illustrated in FIG. 9D has a channel-etched structure in which the oxidesemiconductor film 804 is partly etched between the source electrode 805and the drain electrode 806.

Although the transistor 809 is described as a single-gate transistor, amulti-gate transistor including a plurality of channel formation regionscan be manufactured when a plurality of the gate electrodes 801 which iselectrically connected is included if needed.

Note that the band gap of the oxide semiconductor is 3.0 eV to 3.5 eV.The band gap of silicon carbide and the band gap of gallium nitride are3.26 eV and 3.39 eV, respectively, which are approximately three timesas large as that of silicon. Therefore, these compound semiconductorssuch as silicon carbide and gallium nitride are similar to the oxidesemiconductor in that they are both wide band gap semiconductors. Thecharacteristics of the wide band gap are advantageous for improving thebreakdown voltage of a signal processing circuit, reducing loss ofelectric power, and the like.

However, compound semiconductors such as silicon carbide and galliumnitride are required to be single crystal, and it is difficult to meetthe manufacturing condition to obtain a single crystal material; forexample, crystal growth at a temperature extremely higher than a processtemperature of the oxide semiconductor is needed or epitaxial growthover a special substrate is needed. Such a condition does not allow filmformation of any of these compound semiconductors over a silicon waferthat can be obtained easily or a glass substrate whose allowabletemperature limit is low.

Therefore, an inexpensive substrate cannot be used, and further, thesubstrate cannot be increased in size, so that the productivity ofsignal processing circuits using the compound semiconductor such assilicon carbide or gallium nitride is low. In contrast, since the oxidesemiconductor can be deposited even at a room temperature, the oxidesemiconductor can be deposited over a glass substrate, which leads tohigh productivity.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 7)

In this embodiment, a structure of a transistor which is different fromthat of a transistor formed in accordance with Embodiment 6 will bedescribed.

In FIG. 10A, an example in which a transistor 401 and a capacitor 402are formed over a substrate 400 is illustrated.

The transistor 401 includes, over the substrate 400 having an insulatingsurface, a gate electrode 403; an insulating film 404 over the gateelectrode 403; an oxide semiconductor film 405 which overlaps with thegate electrode 403 with the insulating film 404 provided therebetweenand functions as an active layer; a channel protective film 406 over theoxide semiconductor film 405; and a source electrode 407 and a drainelectrode 408 which are over the oxide semiconductor film 405. Aninsulating film 409 is formed over the oxide semiconductor film 405, thechannel protective film 406, the source electrode 407, and the drainelectrode 408, and the transistor 401 may include the insulating film409 as a component.

In addition, the capacitor 402 includes an electrode 410, the insulatingfilm 404 over the electrode 410, and an electrode 411 over theinsulating film 404.

The channel protective film 406 can be formed by a vapor depositionmethod such as a plasma-enhanced CVD method or a thermal CVD method, ora sputtering method. In addition, the channel protective film 406 ispreferably formed using an inorganic material including oxygen (such assilicon oxide, silicon oxynitride, or silicon nitride oxide). Aninorganic material containing oxygen is used for the channel protectivefilm 406, whereby a structure can be provided, in which oxygen issupplied to a region of the oxide semiconductor film 405 in contact withat least the channel protective film 406 and oxygen deficiency servingas a donor is reduced to satisfy the stoichiometric composition ratioeven when the oxygen deficiency occurs in the oxide semiconductor film405 by heat treatment for reducing moisture or hydrogen. Accordingly,the channel formation region can be made to be an i-type channelformation region or a substantially i-type channel formation region,variation in electrical characteristics of the transistor 401 due tooxygen deficiency can be reduced, and electrical characteristics can beimproved.

Note that a channel formation region corresponds to a region of asemiconductor film, which overlaps with a gate electrode with a gateinsulating film provided between the semiconductor film and the gateelectrode.

The transistor 401 may further include a back gate electrode over theinsulating film 409. The back gate electrode is formed so as to overlapwith a channel formation region in the oxide semiconductor film 405. Theback gate electrode may be electrically insulated and in a floatingstate, or may be in a state where the back gate electrode is suppliedwith a potential. In the case of the latter, the back gate electrode maybe supplied with the same potential as the gate electrode 403, or may besupplied with a fixed potential such as a ground potential. The level ofthe potential supplied to the back gate electrode may be controlled soas to control the threshold voltage of the transistor 401.

In addition, an example in which a transistor 421 and a capacitor 422are formed over the substrate 400, which is different from that in FIG.10A, is illustrated in FIG. 10B.

The transistor 421 includes a gate electrode 423 over the substrate 400having an insulating surface, an insulating film 424 over the gateelectrode 423, a source electrode 427 and a drain electrode 428 whichare over the insulating film 424, and an oxide semiconductor film 425which overlaps with the gate electrode 423 with the insulating film 424interposed therebetween, which is in contact with the source electrode427 and the drain electrode 428, and which serves as an active layer. Aninsulating film 429 is formed over the oxide semiconductor film 425, thesource electrode 427, and the drain electrode 428, and the transistor421 may include the insulating film 429 as a component.

In addition, the capacitor 422 includes an electrode 430, the insulatingfilm 424 over the electrode 430, and an electrode 431 over theinsulating film 424.

The transistor 421 may further include a back gate electrode over theinsulating film 429. The back gate electrode is formed so as to overlapwith the channel formation region in the oxide semiconductor film 425.The back gate electrode may be electrically insulated and in a floatingstate, or may be in a state where the back gate electrode is suppliedwith a potential. In the latter case, the back gate electrode may besupplied with the potential having the same level as the gate electrode423, or may be supplied with a fixed potential such as a groundpotential. By controlling the potential supplied to the back gateelectrode, it is possible to control the threshold voltage of thetransistor 421.

In addition, an example in which a transistor 441 and a capacitor 442are formed over the substrate 400, which is different from that in FIG.10A and that in FIG. 10B, is illustrated in FIG. 10C.

The transistor 441 includes a source electrode 447 and a drain electrode448 over the substrate 400 having an insulating surface, an oxidesemiconductor film 445 serving as active layer over the source electrode447 and the drain electrode 448, an insulating film 444 over the oxidesemiconductor film 445, and a gate electrode 443 which overlaps with theoxide semiconductor film 445 with the insulating film 444 interposedtherebetween. An insulating film 449 is formed over the gate electrode443, and the transistor 441 may include the insulating film 449 as acomponent.

In addition, the capacitor 442 includes an electrode 450, the insulatingfilm 444 over the electrode 450, and an electrode 451 over theinsulating film 444.

Note that it is found that the oxide semiconductor film formed bysputtering or the like includes large amount of impurities such asmoisture or hydrogen. Moisture and hydrogen easily form a donor leveland thus serve as impurities in the oxide semiconductor. Thus, heattreatment is performed on the oxide semiconductor film in an atmosphereof nitrogen, oxygen, ultra-dry air, or a rare gas (argon, helium, or thelike) in order to highly purify the oxide semiconductor film by reducingimpurities such as moisture or hydrogen in the oxide semiconductor film.It is desirable that the content of water in the gas be 20 ppm or less,preferably 1 ppm or less, and more preferably 10 ppb or less. The aboveheat treatment is preferably performed at higher than or equal to 500°C. and lower than or equal to 850° C. (alternatively, a strain point ofa glass substrate or less), more preferably higher than or equal to 550°C. and lower than or equal to 750° C. Note that this heat treatment isperformed at a temperature not exceeding the allowable temperature limitof the substrate to be used. An effect of elimination of moisture orhydrogen by the heat treatment has been confirmed by thermal desorptionspectrometry (TDS).

Note that in a semiconductor display device according to one embodimentof the present invention, a transistor whose channel formation regionincludes an oxide semiconductor is used in a pixel portion, and a drivercircuit may be fabricated using the above-described transistor. In thiscase, the pixel portion and the driver circuit can be formed over onesubstrate.

Alternatively, part or all of a driver circuit may be formed using apolycrystalline semiconductor or single crystal semiconductor which hashigher mobility than an oxide semiconductor, and may be mounted on asubstrate formed in a pixel portion. For example, the transistor formedusing a crystalline semiconductor such as a polycrystalline or singlecrystal semiconductor including silicon, germanium, or the like whosemobility is higher than that of an oxide semiconductor can be formedusing a silicon wafer, an SOI (silicon on insulator) substrate, apolycrystalline semiconductor film over an insulating surface, or thelike.

An SOI substrate can be manufactured using, for example, UNIBOND(registered trademark) typified by Smart Cut (registered trademark),epitaxial layer transfer (ELTRAN) (registered trademark), a dielectricseparation method, a plasma assisted chemical etching (PACE) method, aseparation by implanted oxygen (SIMOX) method, or the like.

A semiconductor film of silicon formed over a substrate having aninsulating surface may be crystallized by a known technique. As theknown technique of crystallization, a laser crystallization method usinga laser beam and a crystallization method using a catalytic element aregiven. Alternatively, a crystallization method using a catalytic elementand a laser crystallization method may be combined. In the case of usinga thermally stable substrate having high heat resistance such as quartz,it is possible to combine any of the following crystallization methods:a thermal crystallization method with an electrically-heated oven, alamp annealing crystallization method with infrared light, acrystallization method with a catalytic element, and a high temperatureannealing method at approximately 950° C.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 8)

A semiconductor display device according to one embodiment of thepresent invention includes a position input device which is referred toas a touch panel.

The touch panel can detect a position which is indicated with a fingeror a stylus in a position detection portion and generate a signalincluding the positional information. Thus, a touch panel is provided sothat the position detection portion overlaps with a pixel portion of apanel, whereby which position of the pixel portion is indicated by auser of the semiconductor display device can be obtained as information.

The position in the position detection portion can be detected by any ofvarious systems such as a resistive system, a capacitance system, anultrasonic system, an optics system including an infrared ray system,and an electromagnetic induction system. FIG. 11 is a perspective viewof a position detection portion using a resistive system. As theposition detection portion of the resistive system, a plurality of firstelectrodes 1630 and a plurality of second electrodes 1631 are providedso that the plurality of first electrodes 1630 is opposed to theplurality of second electrodes 1631 with an interval. When pressingforce is applied to any of the plurality of the first electrodes 1630with a finger or the like, the first electrode 1630 is in contact withany of the plurality of the second electrodes 1631. Then, when a valueof voltage applied to both ends of each of the plurality of the firstelectrodes 1630 and a value of voltage applied to both ends of each theplurality of the second electrodes 1631 are monitored, it is possible tospecify the first electrode 1630 and the second electrode 1631 which arein contact with each other, whereby a position where the finger touchescan be detected.

The first electrode 1630 and the second electrode 1631 can be formedusing a transparent conductive material such as indium tin oxideincluding silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide(ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), forexample.

In addition, a perspective view of the position detection portion usinga projected capacitive system among capacitance systems is illustratedin FIG. 12A. As the position detection portion of the projectedcapacitive system, a plurality of first electrodes 1640 and a pluralityof second electrodes 1641 are provided so that the first electrodes 1640overlap with the second electrodes 1641. Each of the first electrodes1640 has a structure in which a plurality of rectangular conductivefilms 1642 is connected to each other, and each of the second electrodes1641 has a structure in which a plurality of rectangular conductivefilms 1643 is connected to each other. Note that the shapes of the firstelectrodes 1640 and the second electrodes 1641 are not limited to thesestructures.

Further, in FIG. 12A, an insulating layer 1644 functioning as adielectric is provided over and overlaps with the plurality of the firstelectrodes 1640 and the plurality of the second electrodes 1641. In FIG.12B, the plurality of the first electrodes 1640, the plurality of thesecond electrodes 1641, and the insulating layer 1644 which areillustrated in FIG. 12A overlap with one another. As illustrated in FIG.12B, the plurality of the first electrodes 1640 and the plurality of thesecond electrodes 1641 overlap with each other so that the rectangularconductive films 1642 and the rectangular conductive films 1643 aredifferent from each other in position.

When the finger or the like touches the insulating layer 1644, acapacitor is formed between any of the plurality of the first electrodes1640 and the finger, for example. In addition, a capacitor is formedbetween any of the plurality of the second electrodes 1631 and thefinger. Accordingly, when a change of electrostatic capacity ismonitored, it is possible to specify the first electrode 1630 and thesecond electrode 1631 which come the closest to the finger; therefore,the position where the finger touches can be detected.

Note that the touch panel included in the semiconductor display deviceaccording to one embodiment of the present invention may have astructure in which the positional information indicated by the user inthe position detection portion can be taken out as a signal, andstructures other than the structures illustrated in FIG. 11 and FIGS.12A and 12B can be used.

In addition, the liquid crystal display device according to oneembodiment of the present invention may include a photosensor in thepixel portion, instead of the touch panel. An example of a structure ofa pixel portion including a photosensor is schematically illustrated inFIG. 17A.

A pixel portion 1650 illustrated in FIG. 17A includes a pixel 1651 and aphotosensor 1652 corresponding to the pixel 1651. The photosensor 1652includes a transistor and a light-receiving element which has a functionof generating an electrical signal when receiving light, such as aphotodiode. Note that as light which is received by the photosensor1652, reflected light obtained when light from a backlight is shone onan object can be used.

A structure of the photosensor 1652 is illustrated in FIG. 17B. Thephotosensor 1652 illustrated in FIG. 17B includes a photodiode 1653, atransistor 1654, and a transistor 1655. One electrode of the photodiode1653 is connected to a reset signal line 1656, and the other electrodeof the photodiode 1653 is connected to a gate electrode of thetransistor 1654. One of a source electrode and a drain electrode of thetransistor 1654 is connected to a reference signal line 1657, and theother thereof is connected to one of a source electrode and a drainelectrode of the transistor 1655. A gate electrode of the transistor1655 is connected to a gate signal line 1658, and the other of thesource electrode and the drain electrode of the transistor 1655 isconnected to an output signal line 1659.

Note that a circuit configuration of the photosensor 1652 is not limitedto the above-described structure, and may be a circuit configuration inwhich information on light intensity can be taken out as an electricalsignal may be used. In addition, amorphous silicon, microcrystallinesilicon, polycrystalline silicon, or single crystal silicon can be usedfor the photodiode 1653.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

(Embodiment 9)

In a liquid crystal display device according to one embodiment of thepresent invention, a highly reliable transistor with low off-statecurrent is used in a pixel portion; therefore, high visibility and highreliability can be obtained.

FIG. 13 illustrates a cross-sectional view of a pixel in a liquidcrystal display device according to one embodiment of the presentinvention. A transistor 1401 illustrated in FIG. 13 includes a gateelectrode 1402 formed over an insulating surface, a gate insulating film1403 over the gate electrode 1402, an oxide semiconductor film 1404which is over the gate insulating film 1403 and which overlaps with thegate electrode 1402, and a conductive film 1405 and a conductive film1406 which function as a source electrode and a drain electrode andwhich are formed over the oxide semiconductor film 1404. Further, thetransistor 1401 may include an insulating film 1407 formed over theoxide semiconductor film 1404 as a component. The insulating film 1407is formed so as to cover the gate electrode 1402, the gate insulatingfilm 1403, the oxide semiconductor film 1404, the conductive film 1405,and the conductive film 1406.

An insulating film 1408 is formed over the insulating film 1407. Anopening is provided in part of the insulating film 1407 and part of theinsulating film 1408, and a pixel electrode 1410 is formed so as to bein contact with the conductive film 1406 in the opening.

Further, a spacer 1417 for controlling a cell gap of a liquid crystalelement is formed over the insulating film 1408. An insulating film isetched to have a desired shape, so that the spacer 1417 can be formed. Acell gap may also be controlled by dispersing a spherical spacer overthe insulating film 1408.

An alignment film 1411 is formed over the pixel electrode 1410. Further,a counter electrode 1413 is provided in a position opposed to the pixelelectrode 1410, and an alignment film 1414 is formed on the side of thecounter electrode 1413 which is close to the pixel electrode 1410. Thealignment film 1411 and the alignment film 1414 can be formed using anorganic resin such as polyimide or polyvinyl alcohol. Alignmenttreatment such as rubbing is performed on their surfaces in order toalign liquid crystal molecules in certain direction. Rubbing can beperformed by rolling a roller wrapped with cloth of nylon or the likewhile being in contact with the alignment films and the surfaces of thealignment films are rubbed in a certain direction. Note that it is alsopossible to directly form the alignment films 1411 and 1414 that havealignment characteristics by using an inorganic material such as siliconoxide by an evaporation method, without alignment treatment.

Furthermore, a liquid crystal 1415 is provided in a region which issurrounded by a sealant 1416 between the pixel electrode 1410 and thecounter electrode 1413. Injection of the liquid crystal 1415 may beperformed with a dispenser method (dripping method) or a dipping method(pumping method). Note that a filler may be mixed in the sealant 1416.

In FIG. 13, a light-blocking film 1421 which can block light is formedbetween the pixels so that disclination due to variations between thepixels in the alignment of the liquid crystal 1415 is prevented fromseeing. The light-blocking film can be formed using an organic resincontaining a black pigment such as a carbon black or titanium loweroxide. Alternatively, a film of chromium can be used for thelight-blocking film.

Then, a coloring layer 1422 functioning as a color filter, through whichonly visible light in a particular wavelength region is preferentiallytransmitted is provided in a position where the pixel electrode 1410,the counter electrode 1413, and the liquid crystal 1415 overlap witheach other. When the coloring layer 1422 through which light in awavelength region corresponding to red, blue, and green ispreferentially transmitted is provided in each pixel, a full color imagecan be displayed. In this case, it is preferable to use a backlight bywhich white light can be obtained so that color purity of an image isincreased. As the backlight by which white light can be obtained, forexample, a structure in which a red light source, a blue light source,and a green light source are combined; a structure in which a yellow ororange light source and a blue light source are combined; a structure inwhich only a white light source is used; a structure in which a cyanlight source, a magenta light source, and a yellow light source arecombined; or the like can be employed.

Alternatively, light in a wavelength region corresponding to red, blue,and green may be output sequentially from the backlight. In this case, afull color image can be displayed even when a color filter is not used,and luminous efficiency of the semiconductor display device can beincreased. However, when a display state of a display element is fixedat the time of displaying a still image, a single color image instead ofa full color image can be obtained in the case where the color filter isnot used, whereas a full color image can be obtained in the case wherethe color filter is used.

In addition to a cold-cathode tube, a light-emitting element such as anLED or an OLED can be used as a light source. Note that because awavelength of light to be obtained is different depending on a lightsource, a light source may be selected as appropriate in accordance witha required color.

Note that in FIG. 13, the case where the light-blocking film 1421 andthe coloring layer 1422 are provided on the counter electrode 1413 sideis illustrated; however, the light-blocking film 1421 or the coloringlayer 1422 may be provided on the pixel electrode 1410 side. Thepositions of the light-blocking film 1421 and the coloring layer 1422can be set as appropriate in accordance with a direction of lightincident on the liquid crystal 1415 and an emission direction of lighttransmitted through the liquid crystal 1415.

The pixel electrode 1410 and the counter electrode 1413 can be formedusing a transparent conductive material such as indium tin oxideincluding silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide(ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), forexample.

Although a liquid crystal display device of a TN (twisted nematic) modeis described in this embodiment, another liquid crystal display deviceof a VA (vertical alignment) mode, an OCB (optically compensatedbirefringence) mode, an IPS (in-plane-switching) mode, an MVA(multi-domain vertical alignment) mode, or the like may be used.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated within an onlynarrow range of temperature, liquid crystal composition containing achiral agent at 5wt % or more so as to improve the temperature range isused for the liquid crystal 1415. The liquid crystal compositionincluding liquid crystal exhibiting a blue phase and a chiral agent hasa short response time of greater than or equal to 10 μsec and less thanor equal to 100 μsec and is optically isotropic; therefore, alignmenttreatment is not necessary and viewing angle dependence is small.

Note that although the liquid crystal element in which the liquidcrystal 1415 is interposed between a pixel electrode 1409 and thecounter electrode 1413 is described as an example in FIG. 13, the liquidcrystal display device according to one embodiment of the presentinvention is not limited to this structure. A liquid crystal element inwhich a pair of electrodes is provided on one substrate as an IPS typeliquid crystal element or a liquid crystal element using the blue phasemay also be employed.

Next, the appearance of a panel of a liquid crystal display panelaccording to one embodiment of the present invention will be describedwith reference to FIGS. 14A and 14B. FIG. 14A is a top view of a panelin which a substrate 4001 and a counter substrate 4006 are attached toeach other with a sealant 4005, and FIG. 14B is a cross-sectional viewtaken along A-A′ of FIG. 14A.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the substrate4001. The counter substrate 4006 is provided over the pixel portion 4002and the scan line driver circuit 4004. Accordingly, the pixel portion4002 and the scan line driver circuit 4004 are sealed together with aliquid crystal 4007, by the substrate 4001, the sealant 4005, and thecounter substrate 4006.

In addition, a substrate 4021 where a signal line driver circuit 4003 isformed is mounted on the substrate 4001 in a region other than theregion surrounded by the sealant 4005. FIG. 14B illustrates a transistor4009 in the signal line driver circuit 4003.

A plurality of transistors is included in the pixel portion 4002 and thescan line driver circuit 4004, which are formed over the substrate 4001.FIG. 14B illustrates a transistor 4010 and a transistor 4022 which areincluded in the pixel portion 4002. Each of the transistor 4010 and thetransistor 4022 includes an oxide semiconductor in a channel formationregion.

A pixel electrode 4030 included in a liquid crystal element 4011 iselectrically connected to the transistor 4010. A counter electrode 4031of the liquid crystal element 4011 is formed on the counter substrate4006. A portion where the pixel electrode 4030, the counter electrode4031, and the liquid crystal 4007 overlap with each other corresponds tothe liquid crystal element 4011.

A spacer 4035 is provided in order to control the distance (a cell gap)between the pixel electrode 4030 and the counter electrode 4031. Notethat FIG. 14B illustrates the case where the spacer 4035 is formed bypatterning an insulating film; however, a spherical spacer may be used.

Image signals, driving signals, and power supply potentials which areapplied to the signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002 are supplied from a connectionterminal 4016 through lead wirings 4014 and 4015. The connectionterminal 4016 is electrically connected to a terminal of an FPC 4018through an anisotropic conductive film 4019.

Next, FIG. 15 illustrates an example of a perspective view illustratinga structure of the liquid crystal display device according to oneembodiment of the present invention. The liquid crystal display deviceillustrated in FIG. 15 includes a touch panel 1600, a panel 1601, afirst diffuser plate 1602, a prism sheet 1603, a second diffuser plate1604, a light guide plate 1605, a reflective plate 1606, a backlight1607, a circuit board 1608, and a substrate 1611 provided with a signalline driver circuit.

The touch panel 1600, the panel 1601, the first diffuser plate 1602, theprism sheet 1603, the second diffuser plate 1604, the light guide plate1605, and the reflective plate 1606 are sequentially stacked. Thebacklight 1607 is provided at an end portion of the light guide plate1605. Light from the backlight 1607 diffused into the light guide plate1605 is uniformly delivered to the panel 1601 with the help of the firstdiffuser plate 1602, the prism sheet 1603, and the second diffuser plate1604.

The touch panel 1600 includes a position detection portion 1620. Theposition detection portion 1620 is arranged so as to overlap with apixel portion 1621 included in the panel 1601. When a finger, a stylus,or the like touches or gets close to the position detection portion1620, a signal including the positional information is generated.

Note that in the semiconductor display device illustrated in FIG. 15,the touch panel 1600 is arranged between the panel 1601 and the user. Inthis case, when the position detection portion 1620 of the touch panel1600 has light-transmitting properties, the user can see an image in thepixel portion 1621 through the position detection portion 1620. Notethat the touch panel 1600 is not necessarily provided between the panel1601 and the user. For example, in the case where the touch panel 1600is of electromagnetic induction system, the panel 1601 may be providedbetween the user and the touch panel 1600.

Although the first diffuser plate 1602 and the second diffuser plate1604 are used in this embodiment, the number of diffuser plates is notlimited thereto. The number of diffuser plates may be one, or may bethree or more. The diffuser plate may be provided between the lightguide plate 1605 and the panel 1601. Therefore, the diffuser plate maybe provided only on the side closer to the panel 1601 than the prismsheet 1603, or may be provided only on the side closer to the lightguide plate 1605 than the prism sheet 1603.

Moreover, the shape of the cross section of the prism sheet 1603 is notlimited to a sawtooth shape as illustrated in FIG. 15, and may be ashape capable of collecting light from the light guide plate 1605 overthe panel 1601.

The circuit board 1608 is provided with a control circuit for the touchpanel, a CPU, a display control circuit, a control circuit whichcontrols driving of the backlight 1607, and the like. In FIG. 15, thecircuit board 1608 and the panel 1601 are connected to each other via aCOF tape 1609. In addition, the substrate 1611 provided with the signalline driver circuit is connected to the COF tape 1609 by a chip on film(COF) method. In addition, the circuit board 1608 and the touch panel1600 are connected to each other via an FPC 1622.

In FIG. 15, an example is illustrated in which the control circuit whichcontrols driving of the backlight 1607 and the backlight 1607 areconnected to each other via an FPC 1610. However, the above controlcircuit may be formed in the panel 1601; in this case, the panel 1601and the backlight 1607 are connected to each other via an FPC or thelike.

Note that although FIG. 15 illustrates an edge-light type backlightwhere the backlight 1607 is provided on the edge of the panel 1601, theliquid crystal display device of the present invention may be adirect-below type in which the backlight 1607 is provided directly belowthe panel 1601.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

EXAMPLE 1

By using a semiconductor display device according to one embodiment ofthe present invention, an electronic device with low power consumptioncan be provided. In particular, in the case of a portable electronicdevice to which electric power cannot be easily supplied, continuous usetime becomes longer by adding the semiconductor display device accordingto one embodiment of the present invention as a component, which is anadvantage.

The semiconductor display device according to one embodiment of thepresent invention can be used for display devices, laptop personalcomputers, or image reproducing devices provided with recording media(typically, devices which reproduce the content of recording media suchas digital versatile discs (DVDs) and have displays for displaying thereproduced images). Further, the electronic devices in which thesemiconductor display device according to one embodiment of the presentinvention can be used is the following: mobile phones, portable gamemachines, portable information terminals, e-book readers, video cameras,digital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (for example, car audiosystems or digital audio players), copying machines, facsimile machines,printers, versatile printers, automated teller machines (ATMs), vendingmachines, or the like. Specific examples of these electronic devices areillustrated in FIGS. 16A to 16F.

FIG. 16A illustrates an e-book reader including a housing 7001, adisplay portion 7002, and the like. The semiconductor display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 7002. By using the semiconductor display deviceaccording to one embodiment of the present invention for the displayportion 7002, an e-book reader with low power consumption can beprovided. Moreover, when a panel is formed with the use of a flexiblesubstrate and a touch panel has flexibility, the semiconductor displaydevice can have flexibility. Thus, a flexible, lightweight, andeasy-to-use e-book reader can be provided.

FIG. 16B illustrates a display device including a housing 7011, adisplay portion 7012, a supporting base 7013, and the like. Thesemiconductor display device according to one embodiment of the presentinvention can be used for the display portion 7012. By using thesemiconductor display device according to one embodiment of the presentinvention for the display portion 7012, a display device with low powerconsumption can be provided. Note that a display device includes alldisplay devices for displaying information, such as display devices forpersonal computers, for receiving television broadcast, and fordisplaying advertisement, in its category.

FIG. 16C illustrates an automated teller machine including a housing7021, a display portion 7022, a coin slot 7023, a bill slot 7024, a cardslot 7025, a bankbook slot 7026, and the like. The semiconductor displaydevice according to one embodiment of the present invention can be usedfor the display portion 7022. By using the semiconductor display deviceaccording to one embodiment of the present invention for the displayportion 7022, an automated teller machine with low power consumption canbe provided.

FIG. 16D illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, an operation key 7037, a stylus 7038,and the like. The semiconductor display device according to oneembodiment of the present invention can be used for the display portion7033 and the display portion 7034. By using the semiconductor displaydevice according to one embodiment of the present invention for thedisplay portion 7033 and the display portion 7034, a portable gamemachine with low power consumption can be provided. Note that theportable game machine illustrated in FIG. 16D has the two displayportions 7033 and 7034. However, the number of display portions includedin a portable game machine is not limited thereto.

FIG. 16E illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio-input portion 7043, an audio-output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby external images can be loaded. Thesemiconductor display device according to one embodiment of the presentinvention can be used for the display portion 7042. By using thesemiconductor display device according to one embodiment of the presentinvention for the display portion 7042, a mobile phone with low powerconsumption can be provided.

FIG. 16F is a portable information terminal including a housing 7051, adisplay portion 7052, an operation key 7053, and the like. In theportable information terminal illustrated in FIG. 16F, a modem may beincorporated in the housing 7051. The semiconductor display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 7052. By using the semiconductor display deviceaccording to one embodiment of the present invention for the displayportion 7052, a portable information terminal with low power consumptioncan be provided.

This example can be implemented in combination with any of theabove-described embodiments, as appropriate.

This application is based on Japanese Patent Application serial no.2010-102891 filed with the Japan Patent Office on Apr. 28, 2010, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A display device comprising: a pixelportion comprising: a transistor, a channel formation region of thetransistor comprising an oxide semiconductor; and a display elementelectrically connected to the transistor; a driver circuit configured todrive the pixel portion; and a touch panel overlapping with the pixelportion, wherein a driving frequency of the driver circuit is changed inaccordance with an operation signal supplied from the touch panel, andwherein supply of a power supply potential, a clock signal, and a startsignal to the driver circuit is stopped when a still image is displayedin the pixel portion.
 2. The display device according to claim 1,further comprising: a display control circuit electrically connected tothe pixel portion and the driver circuit; and a CPU electricallyconnected to the touch panel and the display control circuit.
 3. Thedisplay device according to claim 1, wherein an off-state currentdensity of the transistor is less than or equal to 100 zA/μm.
 4. Thedisplay device according to claim 1, wherein a length of displaying thestill image in the pixel portion is changed in accordance with theoperation signal supplied from the touch panel.
 5. The display deviceaccording to claim 1, wherein the driving frequency of the drivercircuit when the still image is displayed in the pixel portion is lowerthan the driving frequency of the driver circuit when a moving image isdisplayed in the pixel portion.
 6. The display device according to claim1, wherein the driver circuit is configured to input an image signal tothe pixel portion in a first period when the power supply potential, theclock signal, and the start signal are supplied to the driver circuit,and is configured to stop an input of the image signal to the pixelportion in a second period when supply of the power supply potential,the clock signal, and the start signal to the driver circuit is stopped.7. The display device according to claim 1, wherein the touch panel usesany one of a resistive system, a touch panel capacitance system, anultrasonic system, an optics system, and an electromagnetic inductionsystem.
 8. The display device according to claim 1, wherein the touchpanel comprises a plurality of first electrodes and a plurality ofsecond electrodes overlapping with the plurality of first electrodes. 9.A display device comprising: a pixel portion comprising: a transistor, achannel formation region of the transistor comprising an oxidesemiconductor; a display element electrically connected to thetransistor; and a photosensor; and a driver circuit configured to drivethe pixel portion, wherein a driving frequency of the driver circuit ischanged in accordance with an operation signal supplied from thephotosensor, and wherein supply of a power supply potential, a clocksignal, and a start signal to the driver circuit is stopped when a stillimage is displayed in the pixel portion.
 10. The display deviceaccording to claim 9, further comprising: a display control circuitelectrically connected to the pixel portion and the driver circuit; anda CPU electrically connected to the display control circuit.
 11. Thedisplay device according to claim 9, wherein an off-state currentdensity of the transistor is less than or equal to 100 zA/μm.
 12. Thedisplay device according to claim 9, wherein a length of displaying thestill image in the pixel portion is changed in accordance with theoperation signal supplied from the photosensor.
 13. The display deviceaccording to claim 9, wherein the driving frequency of the drivercircuit when the still image is displayed in the pixel portion is lowerthan the driving frequency of the driver circuit when a moving image isdisplayed in the pixel portion.
 14. The display device according toclaim 9, wherein the driver circuit is configured to input an imagesignal to the pixel portion in a first period when the power supplypotential, the clock signal, and the start signal are supplied to thedriver circuit, and is configured to stop an input of the image signalto the pixel portion in a second period when supply of the power supplypotential, the clock signal, and the start signal to the driver circuitis stopped.
 15. The display device according to claim 9, wherein thephotosensor comprises a photodiode.